Galaxies are not as isolated as they at first glance may seem; on a cosmic scale they congregate in clumps along with dark matter and hot gas. The colourful blob in this new composite image, based on data from several telescopes including ESA's XMM-Newton, is the group of galaxies known as the Bullet Group. Its components appear to be clearly separated, with the hot gas partitioned from the rest of the mass within the group. This is the smallest object ever found to show such an effect, which was caused by a merger in the group's past.
Despite the large distances between them, galaxies rarely exist in isolation. They are mostly found in large assemblies known as groups and clusters. Groups are the smallest gatherings, containing 50 or so galaxies bound together by gravity, whereas clusters are somewhat larger, consisting of hundreds or thousands more.
Dark Matter distribution: