4.3 Dark matter in the Universe
Work on the distribution of galaxies has shown the presence of large
scale structure in the Universe. Large scale flows exist between these
structures, and indicate the presence of dark matter on ever larger
scales. A review of this field has been made by
Dekel (1994).
From a variety of methods, one can infer that in general 
stays
less than 1, but at least as large as 0.2-0.3.
4.3.2 Big Bang nucleosynthesis and the amount of baryons
One of the colloraries of the standard Big Bang model is the
calculation of the nucleosynthesis of the primordial elements.
A first calculation was done in the seventies, and compared
with the observational data on D2, He3,
He4 and Li7.
These calculations and comparison data are now more and more refined,
but recent work still has it that the upper limit to baryonic
dark matter is B
0.02 h-2 with
h 
H0 / (100 km s-1
Mpc-1). (e.g.
Copi et al. 1995).
The difficulties surrounding the determination of the Hubble
Constant need not to be emphasized, but a lower limit for h is 0.4,
and more likely h 
0.75
(Madore et al. 1998).
Thus B is at most
0.13, and more likely about 0.04,
which is lower than the values observed in groups and clusters if
those were typical for the Universe as a whole. It is interesting to
note that for some clusters, like the Coma cluster, the gas fraction
as detected by X-rays alone might exceed the upper bound derived
from Big Bang nucleosynthesis if the Universe is at closure density
(cf. White et al. 1993).
In the early 80's, the most popular scenario for cosmology is the
inflation scenario, which could account for the fact that the observed
matter density in the universe is close to the critical density. In
fact, it was postulated that the matter density, expressed in terms
of the critical density, , is
exactly 1. Thus, compared to the
results from Big Bang nucleosynthesis, there is a lot of dark matter
not in baryonic form, but most likely in the form of a Weakly
Interacting Massive Particle (WIMP). This hypothesis still persists
until today, since it is theoretically very attractive. However,
the actual measurements of the matter density, though difficult, come
out to be ~ 0.2 (cf.
Bahcall et al. 1995),
which
lead some people to think that all of the dark matter could still
be baryonic. Only further work will tell what the real answer is.