X-ray Observations of the Hot Intergalactic Medium

A definite prediction from recent N-body/hydro simulations of the structure formation of the universe is the presence of a diffuse intergalactic medium (IGM) in a temperature range of 10^5 - 10^7 K. This hot phase of the IGM may account for most of the baryon content of the universe and may provide unique information on various physical and chemical processes of the structure formation. I review lines of observational evidence for the hot IGM. The topics include the decomposition of the soft X-ray background into point-like and diffuse components, the preliminary spectroscopic data from ASCA for a thermal component, the separation of the Galactic foreground from the extragalactic background, and the detection of individual hot IGM enhancements near rich clusters of galaxies. The results demonstrate the potential of X-ray observations as a powerful tool to study the large-scale structure of the universe.