Where Are the Baryons?

New, high resolution, large-scale, cosmological hydrodynamic galaxy formationsimulations of a standard cold dark matter model (with a cosmological constant)are utilized to predict the distribution of baryons at the present and atmoderate redshift. It is found that the average temperature of baryons is anincreasing function of time, with most of the baryons at the present timehaving a temperature in the range 10^{5-7} K. Thus, not only is the universedominated by dark matter, but more than one half of the normal matter is yet tobe detected. Detection of this warm/hot gas poses an observational challenge,requiring sensitive EUV and X-ray satellites. Signatures include a soft, cosmicX-ray background, apparent warm components in hot clusters due to bothintrinsic warm intra-cluster gas and warm inter-cluster gas projected ontoclusters along the line of sight, absorption lines in X-ray and UV quasarspectra [e.g., O VI (1032,1038)A lines, OVII 574 eV line], strong emissionlines (e.g., O VIII 653 eV line) and low redshift, broad, low column density$\lya$ absorption lines. We estimate that approximately 1/4 of theextragalactic soft X-ray background (SXRB) (at 0.7 keV) arises from thewarm/hot gas, half of it coming from $z<0.65$ and three-quarters from $z<1.00$,so the source regions should be identifiable on deep optical images.