Thank you for your answers. There are some aspects regarding the distribution of the baryons which are unclear to me. I am not sure whether the following questions fit in this thread. If you want, I could start a new one (or the moderators could split this post). First, lets assume that in the present universe a high fraction of baryons are located in the WHIM. The formation of the WHIM is related to shock waves during gravitational collapse. Shock waves are a part of gravitational collapse, they heat the baryons and influence the efficiency of galaxy formation. Given that shock waves take place already at the very beginning of the gravitational collapse, I assume that only at z > 2 the gravitational potentials are deep enough to generate shock waves that put the baryons in an unbounded (non-virialized) hot phase. On the other hand, another significant fraction of the baryons is located in the intracluster medium (ICM), which does also form at z > 2, more or less (as clusters are forming at z > 2). Naively, I would assume that the intergalactic medium starts collapsing into the galaxy clusters at z ~ 2. Then, some of these baryons collapse to form the ICM and others are shock-heated to form the WHIM. But it is unclear to me whether this picture fits with the observed / assumed metallicities: it would meand that, if the primordial intergalactic medium was very poor in metals, the WHIM should be also very poor in metals and the metal content of the ICM was provided during 2 > z > 0. Is this correct?