Dark Matter and Our Cosmic Web
I’ve always found it fascinating that dark matter has created a sort of scaffolding by which matter in the universe has formed around. Stars, galaxies, clusters and super clusters clump together and are held by this cosmic web. As the universe expands in an ever increasing rate due to dark energy, we see the scaffolding like the galaxies are moving apart from each other. How immense and vast our beautiful universe is. I am often brought to speechless awe when I consider how insignificant our planet, star, and galaxy are when compared to the size of this universe.
In astronomy and cosmology, dark matter is theoretical matter that is inferred to exist from gravitational effects on visible matter and background radiation, but is undetectable by emitted or scattered electromagnetic radiation. Its existence was hypothesized to account for discrepancies between measurements of the mass of galaxies, clusters of galaxies and the entire universe made through dynamical and general relativistic means, and measurements based on the mass of the visible “luminous” matter these objects contain: stars and the gas and dust of the interstellar and intergalactic media.
According to observations of structures larger than galaxies, as well as Big Bang cosmology interpreted under the “Friedmann equations” and the “FLRW metric“, dark matter accounts for 23% of the mass-energy density of the observable universe, while the ordinary matter accounts for only 4.6% (the remainder is attributed to dark energy). From these figures, dark matter constitutes 80% of the matter in the universe, while ordinary matter makes up only 20%.
Dark matter was postulated by Fritz Zwicky in 1934 to account for evidence of “missing mass” in the orbital velocities of galaxies in clusters. Subsequently, other observations have indicated the presence of dark matter in the universe; these observations include the rotational speeds of galaxies, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies.
Dark matter plays a central role in state-of-the-art modeling of structure formation and galaxy evolution, and has measurable effects on the anisotropies observed in the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation. The largest part of dark matter, which does not interact with electromagnetic radiation, is not only “dark” but also, by definition, utterly transparent.
As important as dark matter is believed to be in the universe, direct evidence of its existence and a concrete understanding of its nature have remained elusive. Though the theory of dark matter remains the most widely accepted theory to explain the anomalies in observed galactic rotation, some alternative theoretical approaches have been developed which broadly fall into the categories of modified gravitational laws, and quantum gravitational laws.
Baryonic and nonbaryonic dark matter
A small proportion of dark matter may be baryonic dark matter, astronomical bodies (such as massive compact halo objects) that are composed of ordinary matter, but which emit little or no electromagnetic radiation. However, the vast majority of the dark matter in the universe is believed to be nonbaryonic, and thus not formed out of atoms. It is also believed not to interact with ordinary matter via electromagnetic forces. The nonbaryonic dark matter includes neutrinos, and possibly hypothetical entities such as axions, or supersymmetric particles. Unlike baryonic dark matter, nonbaryonic dark matter does not contribute to the formation of the elements in the early universe (“big bang nucleosynthesis“) and so its presence is revealed only via its gravitational attraction. In addition, if the particles of which it is composed are supersymmetric, they can undergo annihilation interactions with themselves resulting in observable by-products such as photons and neutrinos (“indirect detection”).
Nonbaryonic dark matter is classified in terms of the mass of the particle(s) that is assumed to make it up, and/or the typical velocity dispersion of those particles (since more massive particles move more slowly). There are three prominent hypotheses on nonbaryonic dark matter, called Hot Dark Matter (HDM), Warm Dark Matter (WDM), and Cold Dark Matter (CDM); some combination of these is also possible. The most widely discussed models for nonbaryonic dark matter are based on the Cold Dark Matter hypothesis, and the corresponding particle is most commonly assumed to be a neutralino. Hot dark matter might consist of (massive) neutrinos. Cold dark matter would lead to a “bottom-up” formation of structure in the universe while hot dark matter would result in a “top-down” formation scenario.
The first person to provide evidence and infer the presence of dark matter was Swiss astrophysicist Fritz Zwicky, of the California Institute of Technology in 1933. He applied the virial theorem to the Coma cluster of galaxies and obtained evidence of unseen mass. Zwicky estimated the cluster’s total mass based on the motions of galaxies near its edge and compared that estimate to one based on the number of galaxies and total brightness of the cluster. He found that there was about 400 times more estimated mass than was visually observable. The gravity of the visible galaxies in the cluster would be far too small for such fast orbits, so something extra was required. This is known as the “missing mass problem”. Based on these conclusions, Zwicky inferred that there must be some non-visible form of matter which would provide enough of the mass and gravity to hold the cluster together.
Much of the evidence for dark matter comes from the study of the motions of galaxies. Many of these appear to be fairly uniform, so by the virial theorem the total kinetic energy should be half the total gravitational binding energy of the galaxies. Experimentally, however, the total kinetic energy is found to be much greater: in particular, assuming the gravitational mass is due to only the visible matter of the galaxy, stars far from the center of galaxies have much higher velocities than predicted by the virial theorem. Galactic rotation curves, which illustrate the velocity of rotation versus the distance from the galactic center, cannot be explained by only the visible matter. Assuming that the visible material makes up only a small part of the cluster is the most straightforward way of accounting for this. Galaxies show signs of being composed largely of a roughly spherically symmetric, centrally concentrated halo of dark matter with the visible matter concentrated in a disc at the center. Low surface brightness dwarf galaxies are important sources of information for studying dark matter, as they have an uncommonly low ratio of visible matter to dark matter, and have few bright stars at the center which would otherwise impair observations of the rotation curve of outlying stars.
Gravitational lensing observations of galaxy clusters allow direct estimates of the gravitational mass based on its effect on light from background galaxies. In clusters such as Abell 1689, lensing observations confirm the presence of considerably more mass than is indicated by the clusters’ light alone. In the Bullet Cluster, lensing observations show that much of the lensing mass is separated from the X-ray-emitting baryonic mass.
Galactic rotation curvesRotation curve of a typical spiral galaxy: predicted (A) and observed (B). Dark matter can explain the velocity curve having a ‘flat’ appearance out to a large radius
For 40 years after Zwicky’s initial observations, no other corroborating observations indicated that the mass to light ratio was anything other than unity. Then, in the late 1960s and early 1970s, Vera Rubin, a young astronomer at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington presented findings based on a new sensitive spectrograph that could measure the velocity curve of edge-on spiral galaxies to a greater degree of accuracy than had ever before been achieved. Together with fellow staff-member Kent Ford, Rubin announced at a 1975 meeting of the American Astronomical Society the discovery that most stars in spiral galaxies orbit at roughly the same speed, which implied that their mass densities were uniform well beyond the locations with most of the stars (the galactic bulge). An influential paper presented these results in 1980. These results suggest that either Newtonian gravity does not apply universally or that, conservatively, upwards of 50% of the mass of galaxies was contained in the relatively dark galactic halo. Met with skepticism, Rubin insisted that the observations were correct. Eventually other astronomers began to corroborate her work and it soon became well-established that most galaxies were in fact dominated by “dark matter”:
- Low Surface Brightness (LSB) galaxies. LSBs are probably everywhere dark matter-dominated, with the observed stellar populations making only a small contribution to rotation curves. Such a property is extremely important because it allows one to avoid the difficulties associated with the deprojection and disentanglement of the dark and visible contributions to the rotation curves.
- Spiral Galaxies. Rotation curves of both low and high surface luminosity galaxies appear to suggest a universal density profile, which can be expressed as the sum of an exponential thin stellar disk, and a spherical dark matter halo with a flat core of radius r0 and density ρ0 = 4.5 × 10−2(r0/kpc)−2/3 M⊙pc−3 (here, M⊙ denotes a solar mass, 2 × 1030 kg).
- Elliptical galaxies. Some elliptical galaxies show evidence for dark matter via strong gravitational lensing, X-ray evidence reveals the presence of extended atmospheres of hot gas that fill the dark haloes of isolated ellipticals and whose hydrostatic support provides evidence for dark matter. Other ellipticals have low velocities in their outskirts (tracked for example by planetary nebulae) and were interpreted as not having dark matter haloes. However simulations of disk-galaxy mergers indicate that stars were torn by tidal forces from their original galaxies during the first close passage and put on outgoing trajectories, explaining the low velocities even with a DM halo. More research is needed to clarify this situation.
Note that simulated DM haloes have significantly steeper density profiles (having central cusps) than are inferred from observations, which is a problem for cosmological models with dark matter at the smallest scale of galaxies as of 2008. This may only be a problem of resolution: star-forming regions which might alter the dark matter distribution via outflows of gas have been too small to resolve and model simultaneously with larger dark matter clumps. A recent simulation of a dwarf galaxy resolving these star-forming regions reported that strong outflows from supernovae remove low-angular-momentum gas, which inhibits the formation of a galactic bulge and decreases the dark matter density to less than half of what it would have been in the central kiloparsec. These simulation predictions – bulgeless and with shallow central dark matter profiles – correspond closely to observations of actual dwarf galaxies. There are no such discrepancies at the larger scales of clusters of galaxies and above, or in the outer regions of haloes of galaxies.
Exceptions to this general picture of DM haloes for galaxies appear to be galaxies with mass-to-light ratios close to that of stars. Subsequent to this, numerous observations have been made that do indicate the presence of dark matter in various parts of the cosmos. Together with Rubin’s findings for spiral galaxies and Zwicky’s work on galaxy clusters, the observational evidence for dark matter has been collecting over the decades to the point that today most astrophysicists accept its existence. As a unifying concept, dark matter is one of the dominant features considered in the analysis of structures on the order of galactic scale and larger.
Velocity dispersions of galaxies
In astronomy, the velocity dispersion σ, is the range of velocities about the mean velocity for a group of objects, such as a cluster of stars about a galaxy.
Rubin’s pioneering work has stood the test of time. Measurements of velocity curves in spiral galaxies were soon followed up with velocity dispersions of elliptical galaxies. While sometimes appearing with lower mass-to-light ratios, measurements of ellipticals still indicate a relatively high dark matter content. Likewise, measurements of the diffuse interstellar gas found at the edge of galaxies indicate not only dark matter distributions that extend beyond the visible limit of the galaxies, but also that the galaxies are virialized (i.e. gravitationally bound with velocities corresponding to predicted orbital velocities of general relativity) up to ten times their visible radii. This has the effect of pushing up the dark matter as a fraction of the total amount of gravitating matter from 50% measured by Rubin to the now accepted value of nearly 95%.
There are places where dark matter seems to be a small component or totally absent. Globular clusters show little evidence that they contain dark matter, though their orbital interactions with galaxies do show evidence for galactic dark matter. For some time, measurements of the velocity profile of stars seemed to indicate concentration of dark matter in the disk of the Milky Way galaxy, however, now it seems that the high concentration of baryonic matter in the disk of the galaxy (especially in the interstellar medium) can account for this motion. Galaxy mass profiles are thought to look very different from the light profiles. The typical model for dark matter galaxies is a smooth, spherical distribution in virialized halos. Such would have to be the case to avoid small-scale (stellar) dynamical effects. Recent research reported in January 2006 from the University of Massachusetts, Amherst would explain the previously mysterious warp in the disk of the Milky Way by the interaction of the Large and Small Magellanic Clouds and the predicted 20 fold increase in mass of the Milky Way taking into account dark matter.
In 2005, astronomers from Cardiff University claimed to discover a galaxy made almost entirely of dark matter, 50 million light years away in the Virgo Cluster, which was named VIRGOHI21. Unusually, VIRGOHI21 does not appear to contain any visible stars: it was seen with radio frequency observations of hydrogen. Based on rotation profiles, the scientists estimate that this object contains approximately 1000 times more dark matter than hydrogen and has a total mass of about 1/10th that of the Milky Way Galaxy we live in. For comparison, the Milky Way is believed to have roughly 10 times as much dark matter as ordinary matter. Models of the Big Bang and structure formation have suggested that such dark galaxies should be very common in the universe, but none had previously been detected. If the existence of this dark galaxy is confirmed, it provides strong evidence for the theory of galaxy formation and poses problems for alternative explanations of dark matter.
There are some galaxies whose velocity profile indicates an absence of dark matter, such as NGC 3379. There is evidence that there are 10 to 100 times fewer small galaxies than permitted by what the dark matter theory of galaxy formation predicts. This is known as the dwarf galaxy problem.
Galaxy clusters and gravitational lensing
A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is “bent” around a massive object (such as a cluster of galaxies) between the source object and the observer. The process is known as gravitational lensing.
Dark matter affects galaxy clusters as well. X-ray measurements of hot intracluster gas correspond closely to Zwicky’s observations of mass-to-light ratios for large clusters of nearly 10 to 1. Many of the experiments of the Chandra X-ray Observatory use this technique to independently determine the mass of clusters.
The galaxy cluster Abell 2029 is composed of thousands of galaxies enveloped in a cloud of hot gas, and an amount of dark matter equivalent to more than 1014 Suns. At the center of this cluster is an enormous, elliptically shaped galaxy that is thought to have been formed from the mergers of many smaller galaxies. The measured orbital velocities of galaxies within galactic clusters have been found to be consistent with dark matter observations.
Another important tool for future dark matter observations is gravitational lensing. Lensing relies on the effects of general relativity to predict masses without relying on dynamics, and so is a completely independent means of measuring the dark matter. Strong lensing, the observed distortion of background galaxies into arcs when the light passes through a gravitational lens, has been observed around a few distant clusters including Abell 1689 (pictured right). By measuring the distortion geometry, the mass of the cluster causing the phenomena can be obtained. In the dozens of cases where this has been done, the mass-to-light ratios obtained correspond to the dynamical dark matter measurements of clusters.
A technique has been developed over the last 10 years called weak gravitational lensing, which looks at minute distortions of galaxies observed in vast galaxy surveys due to foreground objects through statistical analyses. By examining the apparent shear deformation of the adjacent background galaxies, astrophysicists can characterize the mean distribution of dark matter by statistical means and have found mass-to-light ratios that correspond to dark matter densities predicted by other large-scale structure measurements. The correspondence of the two gravitational lens techniques to other dark matter measurements has convinced almost all astrophysicists that dark matter actually exists as a major component of the universe’s composition.
The most direct observational evidence to date for dark matter is in a system known as the Bullet Cluster. In most regions of the universe, dark matter and visible material are found together, as expected because of their mutual gravitational attraction. In the Bullet Cluster, a collision between two galaxy clusters appears to have caused a separation of dark matter and baryonic matter. X-ray observations show that much of the baryonic matter (in the form of 107– 108 Kelvin gas, or plasma) in the system is concentrated in the center of the system. Electromagnetic interactions between passing gas particles caused them to slow down and settle near the point of impact. However, weak gravitational lensing observations of the same system show that much of the mass resides outside of the central region of baryonic gas. Because dark matter does not interact by electromagnetic forces, it would not have been slowed in the same way as the X-ray visible gas, so the dark matter components of the two clusters passed through each other without slowing down substantially. This accounts for the separation. Unlike the galactic rotation curves, this evidence for dark matter is independent of the details of Newtonian gravity, so it is held as direct evidence of the existence of dark matter. Another galaxy cluster, known as the Train Wreck Cluster/Abell 520, seems to have its dark matter completely separated from both the galaxies and the gas in that cluster, which presents some problems for theoretical models.
Cosmic microwave background
The discovery and confirmation of the cosmic microwave background (CMB) radiation in 1964 secured the Big Bang as the best theory of the origin and evolution of the cosmos. Since then, many further measurements of the CMB have also supported and constrained this theory, perhaps the most famous being the NASA Cosmic Background Explorer (COBE). COBE found a residual temperature of 2.726 K and in 1992 detected for the first time the fluctuations (anisotropies) in the CMB, at a level of about one part in 105. During the following decade, CMB anisotropies were further investigated by a large number of ground-based and balloon experiments. The primary goal of these experiments was to measure the angular scale of the first acoustic peak of the power spectrum of the anisotropies, for which COBE did not have sufficient resolution. In 2000–2001, several experiments, most notably BOOMERanG found the Universe to be almost spatially flat by measuring the typical angular size (the size on the sky) of the anisotropies. During the 1990s, the first peak was measured with increasing sensitivity and by 2000 the BOOMERanG experiment reported that the highest power fluctuations occur at scales of approximately one degree. These measurements were able to rule out cosmic strings as the leading theory of cosmic structure formation, and suggested cosmic inflation was the right theory.
A number of ground-based interferometers provided measurements of the fluctuations with higher accuracy over the next three years, including the Very Small Array, Degree Angular Scale Interferometer (DASI) and the Cosmic Background Imager (CBI). DASI made the first detection of the polarization of the CMB  and the CBI provided the first E-mode polarization spectrum with compelling evidence that it is out of phase with the T-mode spectrum. COBE’s successor, the Wilkinson Microwave Anisotropy Probe (WMAP) has provided the most detailed measurements of (large-scale)anisotropies in the CMB as of 2009. WMAP’s measurements played the key role in establishing the current Standard Model of Cosmology, namely the Lambda-CDM model, a flat universe dominated by dark energy, supplemented by dark matter and atoms with density fluctuations seeded by a Gaussian, adiabatic, nearly scale invariant process. The basic properties of this universe are determined by five numbers: the density of matter, the density of atoms, the age of the universe (or equivalently, the Hubble constant today), the amplitude of the initial fluctuations, and their scale dependence. This model also requires a period of cosmic inflation. The WMAP data in fact ruled out several more complex cosmic inflation models, though supporting the one in Lambda-CDM amongst others.
In summary, a successful Big Bang cosmology theory must fit with all available astronomical observations (known as the concordance model), in particular the CMB. In cosmology the CMB is explained as relic radiation from the big bang, originally at thousands of degrees kelvin but red shifted down to microwave by the expansion of the universe over the last thirteen billion years. The anisotropies in the CMB are explained as acoustic oscillations in the photon-baryon plasma (prior to the emission of the CMB after the photons decouple from the baryons at 379,000 years after the Big Bang) whose restoring force is gravity. Ordinary (baryonic) matter interacts strongly with radiation whereas, by definition, dark matter does not – though both affect the oscillations by their gravity – so the two forms of matter will have different effects. The power spectrum of the CMB anisotropies shows a large main peak and smaller successive peaks, resolved down to the third peak as of 2009.e.g.. The main peak tells you most about the density of baryonic matter and the third peak most about the density of dark matter (see Cosmic microwave background radiation#Primary anisotropy).
Sky Surveys and Baryon Acoustic Oscillations
The acoustic oscillations in the early universe (see the previous section) leave their imprint in the visible matter by Baryon Acoustic Oscillation (BAO) clustering, in a way that can be measured with sky surveys such as the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey. These measurements are consistent with those of the CMB derived from the WMAP spacecraft and further constrain the Lambda CDM model and dark matter. Note that the CMB data and the BAO data measure the acoustic oscillations at very different distance scales.
Type Ia supernovae distance measurements
Type Ia supernovae can be used as “standard candles” to measure extragalactic distances, and extensive data sets of these supernovae can be used to constrain cosmological models. They constrain the dark energy density ΩΛ= ~0.713 for a flat, Lambda CDM Universe and the parameter w for a quintessence model. Once again, the values obtained are roughly consistent with those derived from the WMAP observations and further constrain the Lambda CDM model and (indirectly) dark matter.
Lyman alpha forest
In astronomical spectroscopy, the Lyman alpha forest is the sum of absorption lines arising from the Lyman alpha transition of the neutral hydrogen in the spectra of distant galaxies and quasars. Observations of the Lyman alpha forest can also be used to constrain cosmological models. These constraints are again in agreement with those obtained from WMAP data.
3D map of the large-scale distribution of dark matter, reconstructed from measurements of weak gravitational lensing with the Hubble Space Telescope.
Dark matter is crucial to the Big Bang model of cosmology as a component which corresponds directly to measurements of the parameters associated with Friedmann cosmology solutions to general relativity. In particular, measurements of the cosmic microwave background anisotropies correspond to a cosmology where much of the matter interacts with photons more weakly than the known forces that couple light interactions to baryonic matter. Likewise, a significant amount of non-baryonic, cold matter is necessary to explain the large-scale structure of the universe.
Observations suggest that structure formation in the universe proceeds hierarchically, with the smallest structures collapsing first and followed by galaxies and then clusters of galaxies. As the structures collapse in the evolving universe, they begin to “light up” as the baryonic matter heats up through gravitational contraction and the object approaches hydrostatic pressure balance. Ordinary baryonic matter had too high a temperature, and too much pressure left over from the Big Bang to collapse and form smaller structures, such as stars, via the Jeans instability. Dark matter acts as a compactor of structure. This model not only corresponds with statistical surveying of the visible structure in the universe but also corresponds precisely to the dark matter predictions of the cosmic microwave background. However, in detail, some issues remain yet to be addressed including an absence of satellite galaxies from simulations and cores of dark matter halos which appear smoother than predicted.
This bottom up model of structure formation requires something like cold dark matter to succeed. Large computer simulations of billions of dark matter particles have been used to confirm that the cold dark matter model of structure formation is consistent with the structures observed in the universe through galaxy surveys, such as the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey, as well as observations of the Lyman-alpha forest. These studies have been crucial in constructing the Lambda-CDM model which measures the cosmological parameters, including the fraction of the universe made up of baryons and dark matter.
Unsolved problems in physics What is dark matter? How is it generated? Is it related to supersymmetry?
Although dark matter was inferred by many astronomical observations, the composition of dark matter remains speculative. Early theories of Dark matter concentrated on hidden heavy normal objects, such as blackholes, neutron stars, faint old white dwarfs, brown dwarfs, as the possible candidates for dark matter, collectively known as MACHOs. Astronomical surveys failed to find enough of these hidden MACHOs. Some hard-to-detect baryonic matter, such as MACHOs and some forms of gas, is believed to make a contribution to the overall dark matter content but would constitute only a small portion.
Additionally, data from a number of lines of evidence, including galaxy rotation curves, gravitational lensing, structure formation, and the fraction of baryons in clusters and the cluster abundance combined with independent evidence for the baryon density, indicate that 85-90% of the mass in the universe does not interact with the electromagnetic force. This “nonbaryonic dark matter” is evident through its gravitational effect. At present, the most common view is that dark matter is primarily non-baryonic, made of one or more elementary particles other than the usual electrons, protons, neutrons, and known neutrinos. The most commonly proposed particles are axions, sterile neutrinos, and WIMPs (Weakly Interacting Massive Particles, including neutralinos).
The dark matter component has much more mass than the “visible” component of the universe. Only about 4.6% of the mass of Universe is ordinary matter. About 23% is thought to be composed of dark matter. The remaining 72% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics. It has been noted that the names “dark matter” and “dark energy” serve mainly as expressions of human ignorance, much like the marking of early maps with “terra incognita“.
An important property of all dark matter is that it behaves like and is modeled like a perfect fluid, meaning that it does not have any internal resistance or viscosity. This means that dark matter particles should not interact with each other other than through gravity, i.e. they move past each other without ever bumping or colliding.
Historically, three categories of dark matter candidates have been postulated. The categories cold, warm, and hot refer to the speed at which the particles are traveling rather than an actual temperature.
- Cold dark matter – objects that move at classical velocities
- Warm dark matter – particles that move relativistically
- Hot dark matter – particles that move ultrarelativistically
Davis et al. wrote in 1985:
Candidate particles can be grouped into three categories on the basis of their effect on the fluctuation spectrum (Bond et al. 1983). If the dark matter is composed of abundant light particles which remain relativistic until shortly before recombination, then it may be termed “hot”. The best candidate for hot dark matter is a neutrino … A second possibility is for the dark matter particles to interact more weakly than neutrinos, to be less abundant, and to have a mass of order 1eV. Such particles are termed “warm dark matter”, because they have lower thermal velocities than massive neutrinos … there are at present few candidate particles which fit this description. Gravitinos and photinos have been suggested (Pagels and Primack 1982; Bond, Szalay and Turner 1982) … Any particles which became nonrelativistic very early, and so were able to diffuse a negligible distance, are termed “cold” dark matter (CDM). There are many candidates for CDM including supersymmetric particles.
Cold dark matter
Cold Dark Matter is dark matter traveling at classical (non-relativistic) speeds. Generally, this is less than 0.1c. This is currently the area of greatest interest for dark matter research, as hot and warm dark matter are not viable theories for galaxy and galaxy cluster formation.
The Concordance model requires that, to explain structure in the universe, it is necessary to invoke cold dark matter. What this cold dark matter can be is completely flexible. They can be large objects like MACHOs or RAMBOs, or particles like WIMPs, axions, etc.
Large masses, like galaxy-sized black holes can be ruled out on the basis of gravitational lensing data. However, tiny black holes are a possibility. Other possibilities involving normal baryonic matter include brown dwarfs or perhaps small, dense chunks of heavy elements; such objects are known as massive compact halo objects, or “MACHOs”. However, studies of big bang nucleosynthesis have convinced most scientists that baryonic matter such as MACHOs cannot be more than a small fraction of the total dark matter.
The DAMA/NaI experiment and its successor DAMA/LIBRA have claimed to directly detect dark matter passing through the Earth, though many scientists remain skeptical since negative results of other experiments are (almost) incompatible with the DAMA results if dark matter consists of neutralinos. Another view is that the DAMA results are evidence that neutralinos might not constitute dark matter, so that scientists should get to work on finding dark matter theories consistent with the experiments.
None of these are part of the standard model of particle physics, but they can arise in extensions to the standard model. In many supersymmetric models naturally give rise to stable dark matter candidates in the form of the Lightest Supersymmetric Particle (LSP); a neutralino is an example of a Supersymmetric particle. Separately, heavy sterile neutrinos exist in non-supersymmetric extensions to the standard model that explain the small neutrino mass through the seesaw mechanism.
Warm dark matter
Warm dark matter are particles traveling at relativistic speeds, less than ultra-relativistic particles, but more than classical particles. Basically, more than 0.1c and less than 0.95c.
Neither hot nor warm dark matter can explain how individual galaxies formed from the Big Bang. That is because hot and warm dark matter move too quickly to be bound to galaxies and thus explain the traditional problems of galactic rotational curves and velocity dispersions that dark matter was postulated to address in the first place. Likewise, hot and warm dark matter moves too quickly to stay together to form the larger-scale structures that can be observed that form weak gravitational lenses (e.g. galaxy clusters).
The microwave background radiation while incredibly smooth, has tiny temperature fluctuations which indicate that matter had clumped on very small scales, which then grew to become the huge galactic clusters and voids seen the in universe today. Fast moving particles, however, cannot clump together on such small scales and, in fact, suppress the clumping of other matter.
There have been no particles discovered so far that can be categorized as warm dark matter. There is a postulated candidate for the warm dark matter category, which is the sterile neutrino: a heavier, slower form of neutrino which doesn’t even interact through the Weak force unlike regular neutrinos. If warm dark matter particles do exist, it would not be enough to explain galactic formation, and cold dark matter would still be required to fill that purpose. Interestingly, some modified gravity theories, such as Scalar-tensor-vector gravity, also require that a warm dark matter exist to make their equations work out.
Hot dark matter are particles that travel at ultra-relativistic velocities. These are approximately velocities over 0.95c.
A known example of hot dark matter already exists: the neutrino. Neutrinos were discovered quite separately from, and long before the search for dark matter seriously began: they were first postulated in 1930, and first detected in 1956. Neutrinos have a very small mass: at least 100000 times less massive than an electron, if not even less than that. Other than gravity, neutrinos only interact with normal matter via the Weak nuclear force making them very difficult to detect (the Weak force only works over the distance of an atomic nucleus, thus a neutrino will only trigger a Weak force event if it hits a nucleus directly head-on). This would classify them as Weakly-Interacting, Light Particles, or WILPs, as opposed to cold dark matter’s theoretical candidates, the WIMPs.
There are three different flavors of neutrinos (i.e. the electron-, muon-, and tau-neutrinos), and their masses are thought to be very close to each other, but still slightly different. The resolution to the solar neutrino problem demonstrated that these three types of neutrinos actually change and oscillate from one flavor to the others and back as they are in-flight. It’s hard to determine an exact upper bound on the collective average mass of the three neutrinos (let alone a mass for any of the three individually). For example, if the average neutrino mass were chosen to be over 50eV / c2 (which is still over 10000 times less massive than an electron), just by the sheer number of them in the universe, the universe would collapse due to their mass. So other observations have served to estimate an upper-bound for the neutrino mass. Using cosmic microwave background data and other methods, it is currently believed that their average mass probably does not exceed 0.3eV / c2. Thus, the normal forms of neutrinos cannot be responsible for the measured dark matter component from cosmology.
Though most gravitational lensing data usually gets explained through cold dark matter theories. Nevertheless, at least one example of lensing data, that of galaxy cluster Abell 1689, can be supported by a light fermionic dark matter in the mass range of few eV — in particular, neutrinos with a mass of about 1.5eV / c2. In this model-fit, active (left-handed) neutrinos account for some 9.5% dark matter with as yet unobserved sterile (right-handed) ones accounting for the rest.
Hot dark matter travels too quickly to be bound by an individual galaxy or a galaxy cluster’s gravity, though a heavy neutrino might be able to affect the shapes of the even larger structures like galaxy superclusters. Thus, hot dark matter is not enough to explain how galaxies form and stay the way they are (e.g. rotation curves). Therefore they would only form a part of the story, and a cold dark matter candidate would still need to be found. Certain theories of modified gravity, such as TeVeS still require neutrino hot dark matter with a certain mass range to make their equations work.
If the dark matter within our galaxy is made up of Weakly Interacting Massive Particles (WIMPs), then a large number must pass through the Earth each second. There are many experiments currently running, or planned, aiming to test this hypothesis by searching for WIMPs. Although WIMPs are a more popular dark matter candidate, there are also experiments searching for other particle candidates such as axions. It is also possible that dark matter consists of very heavy hidden sector particles which only interact with ordinary matter via gravity.
These experiments can be divided into two classes: direct detection experiments, which search for the scattering of dark matter particles off atomic nuclei within a detector; and indirect detection, which look for the products of WIMP annihilations.
An alternative approach to the detection of WIMPs in nature is to produce them in the laboratory. Experiments with the Large Hadron Collider (LHC) may be able to detect WIMPs; because a WIMP has negligible interactions with matter, it may be detected indirectly as (large amounts of) missing energy and momentum which escape the LHC detectors, provided all the other (non-negligible) collision products are detected. These experiments could show that WIMPs can be created, but it would still require a direct detection experiment to show that they exist in sufficient numbers in the galaxy, to account for dark matter.
Direct detection experimentsDirect detection experiments typically operate in deep underground laboratories to reduce the background from cosmic rays. These include: the Soudan mine; the SNOLAB underground laboratory at Sudbury, Ontario (Canada); the Gran Sasso National Laboratory (Italy); the Boulby Underground Laboratory (UK); and the Deep Underground Science and Engineering Laboratory, South Dakota.
The majority of present experiments use one of two detector technologies: cryogenic detectors, operating at temperatures below 100mK, detect the heat produced when a particle hits an atom in a crystal absorber such as germanium. Noble liquid detectors detect the flash of scintillation light produced by a particle collision in liquid xenon or argon. Cryogenic detector experiments include: the Cryogenic Dark Matter Search (CDMS), CRESST, EDELWEISS, EURECA and PICASSO. Noble liquid experiments include ZEPLIN, XENON, DEAP, ArDM, WARP and LUX. Both of these detectors are capable of distinguishing background particles which scatter off electrons, from dark matter particles which scatter off nuclei.
The DAMA/NaI, DAMA/LIBRA experiments have detected an annual modulation in the event rate, which they claim is due to dark matter particles. (As the Earth orbits the Sun, the velocity of the detector relative to the dark matter halo will vary by a small amount depending on the time of year). This claim is so far unconfirmed and difficult to reconcile with the negative results of other experiments assuming that the WIMP scenario is correct.
Directional detection of dark matter is a search strategy based on the motion of the Solar System around the galactic center. By using a low pressure TPC, it is possible to access information on recoiling tracks (3D reconstruction if possible) and to constrain the WIMP-nucleus kinematics. WIMPs coming from the direction in which the Sun is travelling (roughly in the direction of the Cygnus constellation) may then be separated from background noise, which should be istropic. Directional dark matter experiments include DMTPC, DRIFT, Newage and MIMAC.
On 17 December 2009 CDMS researchers reported two possible WIMP candidate events. They estimate that the probability that these events are due to a known background (neutrons or misidentified beta or gamma events) is 23%, and conclude “this analysis cannot be interpreted as significant evidence for WIMP interactions, but we cannot reject either event as signal.”
Indirect detection experiments
Indirect detection experiments search for the products of WIMP annihilation. If WIMPs are majorana particles (the particle and antiparticle are the same) then two WIMPs colliding would annihilate to produce gamma rays, and particle-antiparticle pairs. This could produce a significant number of gamma rays, antiprotons or positrons in the galactic halo. The detection of such a signal is not conclusive evidence for dark matter, as the backgrounds from other sources are not fully understood.
The EGRET gamma ray telescope observed an excess of gamma rays, but scientists concluded that this was most likely a systematic effect. The Fermi Gamma-ray Space Telescope, launched June 11, 2008, is searching for gamma rays events from dark matter annihilation. At higher energies, the ground-based MAGIC gamma-ray telescope has set limits to the existence of dark matter in dwarf spheroidal galaxies  and clusters of galaxies .
The PAMELA payload (launched 2006) has detected an excess of positrons, which could be produced by dark matter annihilation, but may also come from pulsars. No excess of anti-protons has been observed.
WIMPs passing through the Sun or Earth are likely to scatter off atoms and lose energy. This way a large population of WIMPs may accumulate at the center of these bodies, increasing the chance that two will collide and annihilate. This could produce a distinctive signal in the form of high energy neutrinos originating from the center of the Sun or Earth. It is generally considered that the detection of such a signal would be the strongest indirect proof of WIMP dark matter. High energy neutrino telescopes such as AMANDA, IceCube and ANTARES are searching for this.
Although dark matter is the most popular theory to explain the various astronomical observations of galaxies and galaxy clusters, direct observational evidence of dark matter has remained elusive. Some alternative theories have been proposed to explain these observations without the need for a vast amount of undetected matter. They broadly fall into the categories of modified gravity laws, and quantum gravity laws. The difference between modified gravity laws and quantum gravity laws is that modified gravity laws simply propose alternative behaviour of gravity at astrophysical and cosmological scales, without any regard to the quantum scale. Both posit that gravity behaves differently at different scales of the universe, making the laws established by Newton and Einstein insufficient.
Modified gravity laws
One group of alternative theories to dark matter assume that the observed inconsistencies are due to an incomplete understanding of gravitation rather than invisible matter. These theories propose to modify the laws of gravity instead.
The earliest modified gravity model to emerge was Milgrom’s Modified Newtonian Dynamics or MOND in 1983, which adjusts Newton’s laws to create a stronger gravitational field when gravitational acceleration levels become tiny (such as near the rim of a galaxy). It had some success in predicting galactic-scale features, such as rotational curves of elliptical galaxies, and dwarf elliptical galaxies, etc. It fell short in predicting galaxy cluster lensing. However, MOND was not relativistic, since it was just a straight adjustment of the older Newton’s gravitational laws, not of the newer Einstein’s laws. Work began soon after to make MOND conform to relativity. It’s an ongoing process, and many competing theories have emerged based around the original MOND theory, such as TeVeS, and MOG or STV gravity, etc.
In 2007, John W. Moffat proposed a modified gravity theory based on the Nonsymmetric Gravitational Theory (NGT) that claims to account for the behavior of colliding galaxies. This theory still requires the presence of non-relativistic neutrinos, other candidates for (cold) dark matter, to work.
Another proposal utilizes a gravitational backreaction in an emerging theoretical field that seeks to explain gravity between objects as an action, a reaction, and then a back-reaction. Simply, an object A affects an object B, and the object B then re-affects object A, and so on: creating somewhat of a feedback loop that strengthens gravity.
Recently, another group has proposed a modification of large scale gravity in a theory named “dark fluid“. In this formulation, the attractive gravitational effects attributed to dark matter are instead a side-effect of dark energy. Dark fluid combines dark matter and dark energy in a single energy field that produces different effects at different scales. This treatment is a simplified approach to a previous fluid-like model called the Generalized Chaplygin gas model where the whole of spacetime is a compressible gas. Dark fluid can be compared to an atmospheric system. Atmospheric pressure causes air to expand, but part of the air can collapse to form clouds. In the same way, the dark fluid might generally expand, but it also could collect around galaxies to help hold them together.
Another set of proposals is based on the possibility of a double metric tensor for space-time. It has been argued that time reversed solutions in general relativity require such double metric for consistency, and that both Dark Matter and Dark Energy can be understood in terms of time reversed solutions of general relativity.
Quantum Gravity is an active wide-ranging theoretical physics field that encompasses many different competing theories, and even many different competing families of theories. It is also sometimes known as the Theory of Everything or TOE. Basically, it is a class of theories that attempts to reconcile the two great not-yet-reconciled laws of physics, gravitation with quantum mechanics, and obtain corrections to the current gravitational laws. Examples of quantum gravity theories are Superstring theory, its successor M-Theory, as well as the competing Loop Quantum Gravity.
In a sense, quantum gravity is a much more ambitious field of study than dark matter, since quantum gravity is an all-encompassing attempt to reconcile gravity with the other fundamental forces of nature, whereas dark matter is simply a classical physics solution for a classical gravity problem. It is hoped that once a testable quantum gravity theory emerges, that one of its side benefits will be to explain these various gravitational mysteries from first principles rather than through empirical methods alone.
Some Superstring/M-Theory cosmologists propose that multi-dimensional forces from outside the visible universe have gravitational effects on the visible universe meaning that dark matter is not necessary for a unified theory of cosmology. M-Theory envisions that the universe is made up of more than the 3 spatial and 1 time dimensions that we all are used to, that there are up to 11 dimensions altogether. The remaining dimensions are hidden from our full view and only show up at the quantum levels. However, if there are particles or energy that exist only within these alternate dimensions, then they might account for the gravitational effects currently attributed to dark matter.
Loop quantum gravity and its subset Loop quantum cosmology envisions spacetime itself as being made up of elementally small particles, or quanta. This is quite different from how we usually envision empty space, as being simply empty, i.e. full of nothing: LQG and LQC says even empty space is actually made of something. Each particle of spacetime in various ways loops up (combines and twists) with adjacent particles of spacetime to create all of the matter and energy we see in the universe today. In this sense, if matter is just crumpled up spacetime, then even the empty untwisted space near a large body of matter would be put under more tension than empty untwisted space far away from matter; think of a long chain that you crumple up in the middle, the uncrumpled chainlinks near the crumpled up portion would still feel a large tension. This can be thought of as the same effect as dark matter. Chain links far away from the twists would feel little or no tension and would be in a state of relaxation, this can be analagous to dark energy.
In a 2004 study at the University of Mainz in Germany, it has been found that if one applies just a standard quantum mechanical approach to Newton’s Gravitational constant at various scales within the astrophysical realm (i.e. scales from solar systems up to galaxies), it can be shown that the Gravitational constant is not so constant anymore and actually starts to grow. The implication of this is that if the Gravitational constant grows at different scales, then dark matter is not needed to explain galactic rotational curves.