The
warm–hot intergalactic medium (
WHIM) refers to a sparse, warm-to-hot (10
5 to 10
7 K) plasma that
cosmologists believe to exist in the spaces between
galaxies and to contain 40–50% of the
baryons (that is, 'normal matter' which exists as
plasma or as
atoms and
molecules, in contrast to
dark matter) in the universe at the current
epoch. Because of the high temperature of the medium, the expectation is that it is most easily observed from the ultraviolet and low energy X-ray emission.