Most of Ordinary Matter in the Universe is in Warm/Hot Gas

New, high resolution, large-scale, cosmological hydrodynamic galaxy formation simulations of a standard %COBE and cluster normalized cold dark matter model (with a cosmological constant) are utilized to predict the distribution of baryons at the present and 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 $10^{5-7}$K. Thus, not only the universe is 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 observational challenge, requiring sensitive EUV and X-ray satellites. Signatures include a soft cosmic X-ray background, apparent warm components in hot clusters due to both intrinsic warm intra-cluster gas and warm inter-cluster 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, OVII 574eV line], strong emission lines (e.g., O VIII 653 eV line) and low redshift, broad, low column density $\lya$ absorption lines.