New high-resolution, large-scale cosmological hydrodynamic galaxy formation
simulations of a standard cold dark matter model (with a cosmological cons
tant) are utilized to predict the distribution of baryons at the present an
d at moderate redshift. It is found that the average temperature of baryons
is an increasing function of time, with most of the baryons at the present
time having a temperature in the range of 10(5)-10(7) K. Thus not only is
the universe dominated by dark matter, but more than one-half of the normal
matter is yet to be detected. Detection of this warm/hot gas poses an obse
rvational challenge, which requires sensitive EUV and X-ray satellites. Sig
natures include a soft cosmic X-ray background, apparent warm components in
hot clusters due to both intrinsic warm intracluster and intercluster gas
projected onto clusters along the line of sight, absorption lines in X-ray
and UV quasar spectra [e.g., O VI(1032, 1038) A lines, O VII 574 eV line],
strong emission lines (e.g., O Mn 653 eV line), and low-redshift, broad, lo
w column density Ly alpha absorption lines. We estimate that approximately
one-fourth of the extragalactic soft X-ray background (at 0.7 keV) arises f
rom the warm/hot gas, half of it coming from z < 0.65, and three-quarters c
oming from z < 1.00, so the source regions should be identifiable on deep o
ptical images.