The existence of large amounts of Dark Matter in the universe, manifesting itself through its gravitational effects, is a well established fact, although the precise amount has been a matter of a lively debate through the years. Attempts to identify the DM as normal baryonic matter has failed, mostly due to the nucleosynthesis constraints imposed by the successful hot Big-Bang model and the large temperature fluctuations of the CMB that it predicts in a flat Universe (T / T ~ 1/3 / ). Possibly some of the DM could be neutral hydrogen, in the form of Lyman- clouds, but it is estimated that it could contribute only 0.01. Similarly, the possible solid form of baryonic material (eg. dust grains, Jupiters, dwarfs with M 0.08 M or neutron stars) would contribute little to .
Two recent determinations of the deuterium abundance, which combined with the BBN (Big-Bang Nucleosynthesis) predictions, give the total baryonic mass in the universe, have provided slightly discrepant results (see reviews , ), covering the range:
and therefore for h = 0.72 we have: 0.01 b 0.046. However, the recent analysis of the results from the BOOMERANG CMB experiment have provided a value mostly compatible with the lower deuterium abundance and thus higher b h2 value (see further below).
In this section we present a variety of methods used to estimate either the total mass/energy density of the Universe or its mass density, m.