b
The value that we get of
b, gal
~ 0.003 in galaxies is only about one-tenth of the
value of the baryonic density of the universe
b ~ 0.03
derived from measurements of light element abundances
([Coc
et al. 2004])
and from the shape of the angular power spectrum of the cosmic microwave
background
[Spergel
et al. 2003].
This opens up the important question: if they are not in galaxies, where
do 90% of the baryons in the Universe reside?
Hydrodynamic simulations ([Cen & Ostriker 1999]) suggest that most of these baryons are in a warm/hot intergalactic medium (WHIM), that is highly ionised. This medium is very difficult to detect, but there might be indirect evidence for its existence, from observations of OVII X-ray absorption along the lines of sight to high redshift quasars (e.g. [Fang et al. 2003]). Directly detecting the WHIM and will be a major area of observational study over the next few years.