The arguments that constrain the nature of the DM are nearly as varied as the DM candidates that have been proposed over the years. Similarly, attempts to detect DM involve a wide range of experimental techniques. The field has grown to such an extent that a recent major workshop was devoted entirely to baryonic DM (Lynden-Bell and Gilmore 1990), while discussions of experiments to detect non-baryonic DM candidates are assigned a good deal of time at cosmology meetings. The treatment given here of some of these issues is necessarily brief and an emphasis is placed on the galactic aspects of the problem.
Perhaps the most fundamental question concerning the DM in galaxies is
whether it is baryonic or non-baryonic. Standard primordial nucleosynthesis
calculations indicate that the baryonic density of the Universe
b is around
0.015h-2
(Walker et al. 1991;
Peebles et al. 1991).
The most recent study incorporates a Monte Carlo
analysis to estimate the uncertainties in the predicted abundances of light
elements
(Smith, Kawano and
Malaney 1993).
When compared with observations, this work yields:
although inhomogeneities in the early universe allow a somewhat greater range
(see below).
According to
Efstathiou, Ellis and
Peterson (1988),
the mass-to-light ratio of
galaxies is related to the density parameter by
Thus for typical galactic mass-to-light ratios summarized in previous
sections, all the DM in galaxies could be baryonic.
These results also indicate that much of the baryonic
material in the Universe remains undetected (see also
Carr 1990;
Walker et al. 1991;
Persic and Salucci
1992).
However,
these baryons need not make up the DM in galactic halos, but could
instead be in a uniformily distributed intergalactic medium. Thus
nucleosynthesis calculations tell us nothing about DM in galaxies other
than it could be baryonic, non-baryonic, or a mixture of the two.
