  We point out that two problems of observational cosmology, the facts i) that
> 60% of the baryonic content of the universe is not observed at z=0 and ii)
that the properties of small clusters do not agree with simple expectations,
could be closely related. As shown by recent studies, the shock-heating
associated with the formation of large-scale structures heats the intergalactic
medium (IGM) and leads to a ``warm IGM'' component for the gas. In the same
spirit, we suggest the intracluster medium (ICM) to be a mixture of
galaxy-recycled, metal enriched gas and intergalactic gas, shock-heated by the
collapsing much larger scales. This could be obtained through two processes: 1)
the late infalling gas from the external warm IGM is efficiently mixed within
the halo and brings some additional entropy, or 2) the shocks generated by
larger non-linear scales are also present within clusters and can heat the ICM.
We show that if assumption (1) holds, the entropy brought by the warm IGM is
sufficient to explain the observed properties of clusters, in particular the
entropy floor and the LX-T relation. On the other hand, we briefly note that
the scenario (2) would require a stronger shock-heating because of the larger
density of the ICM as compared with filaments. Our scenario of clusters being
"born warm" can be checked through the predicted redshift evolution of the
entropy floor.
