  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.
