Although it cannot be seen, there is solid evidence that more than 90% of the total mass in the universe is composed of Dark Matter. It is there! It is there! It is simply not visible. This unseen mass has been detected through several astrophysical methods, beginning with the measurements in spiral galaxies of what are called “rotation curves,” where the speed of an object in its orbital path at one place in a galaxy is measured and compared with the speeds of other orbital movement curves farther out in the spiral arms of that galaxy. Since the measured speeds of all of the so-called “rotation curves” have turned out to be surprisingly similar to each other, even at different distances from the center of the galaxy, it follows that something is distinctly unusual about these orbital speeds. These rotational speeds have not followed the time-honored astrophysical models for movement of astronomical bodies such as planets in our own Solar System where the speeds of the planets get progressively slower the farther the planets are from the Sun. What exactly is happening here in the spiral galactic model? Is there extra unseen mass which is causing the stars, etc., to maintain their orbital speed on their paths around the Milky Way, even far out from the center of the Galaxy?
In other important studies, astrophysicists have measured the mass of the gas in and around elliptical galaxies and have additionally measured the movement of galaxies in galactic clusters, detecting what they call the “velocity dispersion” or the spreading out of the velocities (directional speeds) of galaxies orbiting around the centers of the galactic clusters. The velocities of the galaxies in the galactic clusters do not seem to slow down as expected as the individual galaxies get farther out from the centers of these galactic clusters. It is very puzzling for astrophysicists. This should not be! But, regardless, it is.
Additionally, astrophysicists have measured the X-ray gas in the clusters of galaxies. They have scrutinized and analyzed the gravitational lensing - the arcs of light showing the presence of a massive, but dark, body hiding a bright object which is directly behind that massive body. Using time-honored methods of calculating mass and expected velocity dispersion astrophysicists have found that there is much more mass than can be observed directly. Because the Dark Matter cannot be seen directly, the measured hidden massive objects may be composed of different material than ordinary baryons (hereinafter “stuff”). The total density of the invisible (often erroneously referred to as “missing”) mass appears to be much greater than the total density of “stuff” in the universe as has been calculated using the most likely abundances in the early universe of the light elements (hydrogen, helium, deuterium, and lithium). Dark Matter, in the form of nonbaryons (hereinafter “non stuff”), seems to be an obvious explanation for the formation of galactic clusters, as shown by the observed temperature fluctuations presently seen in the cosmic microwave background radiation (CMBR), which is the radiation recorded at 2.7 degrees Kelvin left over from the Big Bang.
In addition, the CMBR gives evidence of fluctuations in density which, because of gravitational instability, can only relate to the present structure if it is “non stuff.” Because the CMBR possesses gravitational instability due to the observed fluctuations in density, it would seem that Omega (calculated as the density of the universe divided by what is called the “critical density of the universe”) is very likely equal to 1.
There are three possibilities in determining the Effective Omega (the density parameter of the universe divided by the critical density of the universe):
If the universe turns out to be composed only of “stuff,” the amount of this “stuff” will be equal to the combined total of all of the “stuff” in the universe. Some of this unseen “stuff” will be found in what are called MACHOs (Massive Astrophysical Compact Halo Objects) which are made up of the familiar “stuff” but which are invisible to detectors, because the “stuff” does not emit light. Some of this unseen “stuff” may consist of difficult-to-detect neutral Hydrogen atoms which have recently been found in great abundance on the outskirts of galaxies.
However, if this added material is “non stuff,” then there is a zoo of exotic particles waiting in the wings! The most probable “non stuff” candidates for the Dark Matter particles are: Axions (hypothetical elementary particles having low mass and zero charge), neutrinos (which are now thought to have a tiny, near vanishing, mass but no charge and which have been detected as “oscillating” - changing from one kind (”flavor”) of neutrino to another kind (”flavor”) of neutrino), and/or WIMPs (Weakly Interacting Massive Particles). And there are others. There are many, many others.
Considering the nature of Dark Matter:
Dark Matter (DM) has basic problems. There are probably not enough MACHOs (consisting of “stuff”). Therefore, “non stuff” particles, such as the Cosmic Microwave Background Radiation (CMBR), probably must be added to make up some of the observed mass of the universe. The best additional candidates for Dark Matter are Axions, which are as yet hypothetical. Another candidate for the Dark Matter is the neutrino. At least one type (”flavor”) of neutrino (which at one time was hypothetical) has recently been recorded as having an indeterminate, but extremely tiny, mass difference, which implies that at least one of the neutrino types (”flavors”) has mass, even if nobody knows which type (”flavor”) of neutrino is involved or the actual mass of that type (”flavor”) of neutrino. Finally, there are the hypothetical WIMPs. Obviously, being hypothetical, WIMPs are yet to be detected.
It should be emphasized that cosmology is joining the list of experimental sciences by using many techniques, including astronomy, astrophysics, particle physics, nuclear physics, condensed matter physics, low temperature physics, you-name-it physics, etc. The list continues to expand. Where will it all end? Particle Astrophysics has taken center stage. The suspense is almost too much to bear.