Where are the baryons?

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.