The intergalactic medium and soft X-ray background

I present an overview of some of the recent advances in our understanding of the distribution and evolution of the ordinary, baryonic matter in the universe. Two observations that strongly suggest that most of the baryons seen at high redshift (approximate to greater than or equal to 2) have turned into some forms Set undetected. at approximate to = 0 are highlighted. With the aid of large-scale cosmological hydrodynamic simulation. it is shown that most of the baryons today are in a gaseous form with a temperature of 10(5) - 10(7) Kelvin - the ''warm/hot gas", shock heated during the gravitational collapse and formation of the large-scale structure at Loa redshift. Primarily line emissions from this warm/hot gas may account for a large fraction of the residual (after removal of identifiable discrete sources) soft Xray background at hv < 1.0keV. How this warm/hot gas mag be detected by the next generation of EUV and soft X-ray instruments is indicated, Detection or non-detection of this warm/hot gas will have profound implication for cosmology.