Objects which emit light, whether they are cigars, light bulbs, stars or galaxies can be characterized by their emitted energy per unit mass. This is parameterized as the Mass to Luminosity ratio or M / L. For cosmological purposes, it is most convenient to express M / L in terms of solar masses and luminosities. For main sequence stars it can be shown that L Mn where n 3.5-4. Thus a 10 M star has M / L 10-3, a 1 M star has M / L =1 and a 0.1 M star has M / L 1000. The term "dark matter" refers to the the existence of objects which have extreme M / L. Identifying the nature and extent of the dark matter component in the Universe is, arguably, the most significant unsolved problem in all of cosmology.
The cosmological parameter , of course, is insensitive to the nature of the mass in the Universe. The only requirement is that this mass gravitates. This requirement can be meet if the dark matter is composed of a small space density of very massive objects or a large space density of very low mass objects. It is the total integration of this mass-density over spacetime that determines and hence the curvature of the Universe. Some clues about the nature of dark matter can be obtained by determining its distribution. For instance, if it can be shown that the galaxy distribution is an unbiased tracer of the mass distribution, then we can conclude that the dark matter is exclusively associated with galaxies and is not found between galaxies. In this case, the dark matter must be of a form which allows it to dissipate into small scale gravitational potentials. At the other extreme, if there is significant bias such that most of the mass is distributed between the galaxies, then it is not prone to clumping on small scales and therefore is distributed more diffusely. In general, the evidence for dark matter is a result of analyzing the motions of test particles on some scale and then applying the virial theorem to estimate the mass. If the virial mass is larger than the mass estimated from the "light" then this indicates the presence of gravitating mass which has no corresponding light and is therefore "dark". The ability of dark matter to gravitate is independent of its nature. A test particle under the influence of gravity will not care whether the gravotating mass is baryonic or nonbaryonic. Figure 4-1 provides an overview of the possible kinds of dark matter which could exist and provide most of the gravitational mass in the Universe.
Figure 4-1: Flow chart overview of the Dark Matter situation. Possible candidates are driven by what the actual value of Omega is. The role of unknown physics in all of this is indicated by the dashed lines and boxes.