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 [107], [163]), covering the range:
| (63) |
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.