The inventory of cosmic baryons is readily compiled. Stars and their
remnants, and gas in galaxies, contribute no more than 1% of the
critical density (i.e. they give
b <
0.01). However several percent more
could be contributed by diffuse material pervading intergalactic space:
warm gas (with
kT 
0.1 keV) in
groups of galaxies and loose clusters, and cooler gas
pervading intergalactic space that manifests itself via the `picket fence'
absorption lines in quasar spectra. (Rich clusters are rare, so their
conspicuous gas content, at several KeV, is not directly significant for
the total inventory, despite its importance as a probe)
These baryon estimates are concordant with those inferred by matching
the He and D abundances at the birth of galaxies with the predicted outcome
of nucleosynthesis in the big bang, which is sensitive to the primordial
baryon/photon ratio, and thus to
b. The
observational estimates have
firmed up, with improved measurements of deuterium in high-z
absorbing clouds. The best fit occurs for
b
0.02h-2 where h is the Hubble constant in units
of 100 km s-1 Mpc-1. Observations favour
h 0.7.
b is now
pinned down by a variety of argument to be
0.04 - 0.05. This corresponds to only ~ 0.3 baryons per cubic metre, a
value so low that it leaves little scope for dark baryons. (It is
therefore unsurprising that the MACHO/OGLE searches should have found that
compact objects do not make a substantial contribution to the total mass
of our own galactic halo.)