X-Ray satellites have
found that clusters
of galaxies are immersed in hot gas that is a product of galaxy formation
and that emits strongly in the X-Ray portion of the spectrum. These are called
X-ray clusters of galaxies. The adjacent figure shows an Einstein
X-Ray Observatory image of X-Rays from the Perseus Cluster of galaxies.
Because they are such bright X-Ray sources, these clusters can be seen at
cosmological distances and are used extensively to map the large-scale structure
of the Universe.
Information supplied by: http://csep10.phys.utk.edu