  The aim of this paper is to present the case that stellar mass primordial
black holes make up the dark matter component of the Universe. A near critical
density of compact bodies implies that most lines of sight will be
gravitationally microlensed, and the paper focuses on looking for the predicted
effects on quasar brightness and spectral variations. These signatures of
microlensing include the shape of the Fourier power spectrum of the light
curves, near achromatic and statistically symmetric variations, and the absence
of time dilation in the timescale of variability. For spectral changes, as the
continuum varies there should be little corresponding change in the strength of
the broad lines. In all these cases, the observations are found to be
consistent with the predictions for microlensing by a population of stellar
mass compact bodies. For multiply lensed quasars where the images vary
independently it is shown that stellar populations are too small to produce the
observed microlensing effects, implying a population of compact dark matter
bodies of around a stellar mass along the line of sight. The most serious
objection to dark matter in the form of compact bodies has come from
observations of microlensing of stars in the Magellanic Clouds. In this paper
the expected event rate is re-analysed using more recent values for the
structure and dynamics of the Galactic halo, and it is shown that there is then
no conflict with the observations. Finally, the possible identity of a near
critical density of dark matter in the form of stellar mass compact bodies is
reviewed, with the conclusion that by far the most plausible candidates are
primordial black holes formed during the QCD epoch. The overall conclusion of
the paper is that primordial black holes should be seen alongside elementary
particles as viable dark matter candidates.
