Among many surprising facts of astronomy is that stars contain a negligible fraction of the total mass in the universe. Most mass (~80%) is in the form of the mysterious dark matter. The minority baryons (comprising rest ~20% of the mass) are distributed in different forms (http://arxiv.org/abs/astro-ph/9712020). At the current redshift most baryonic mass is in the X-ray halos of groups and clusters of galaxies; followed by mass in stars (most of the stars are in spheriodal components of galaxies); and by neutral and molecular gas in galaxies. Groups, the biggest baryon reservoirs, are diffuse and extremely difficult to observe in X-rays and therefore their baryon content is quite uncertain.
At high redshift (z~3) the conditions are rather different. The cosmic density is much higher and thus the cooling time is shorter. Moreover, almost all halos are less massive than 1.e11 solar masses, and hence are cooling efficiently. At that redshift most of the baryons are in the form of neutral Hydrogen clouds embedded within ~ 100s of kpc from galaxies. These `clouds' show up in Ly-alpha absorption of background quasars. Some of these clouds are optically thick and are known as damped Ly-alpha absorber (DLAs). The total mass estimated in these clouds is roughly equal to the total baryonic mass at current redshifts. Comparing the baryonic distribution at z=3 and z=0 then suggests that most Ly-alpha absorbing clouds (and stellar outflows generated by stars formed due to the cooling of these filaments) are shock-heated to X-ray temperatures at z=0 and fill the groups and clusters of galaxies.
Moreover, the total baryonic mass fraction at z=0 and z=3 agrees with the baryonic mass fraction calculated from big-bang nucleosynthesis. This implies that most baryonic mass in the universe is in detectable form; this is a good news for us astrophysicists as we can try to account for all of it.
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