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Most of the galaxies in the universe are found in CLUSTERS bound together
by gravitational forces. In the visible band
optical photographs of the centre of
the Coma cluster, show large numbers of galaxies close together on the
sky. When we look in the same direction with an X-ray telescope the
appearance is radically different as is shown in the accompanying ROSAT
image. Instead of the individual galaxies we see a enormous cloud of hot
gas filling the whole cluster. The gas itself was heated as it fell into
the cluster under the influence of gravity, reaching temperatures of
around 50 million degrees, guaranteeing emission at X-ray wavelengths.
By studying the X-ray emission from clusters of galaxies like Coma we can
measure the amount of hot gas involved and hence deduce how much gravitational
mass is needed. X-ray observations provide the best evidence we have that a
large fraction of the gravitational mass must be in the form of "dark matter".