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. Here is a link to computer simulations of X-Ray cluster formation.