galaxy cluster

Galaxies are not sprinkled evenly through space; they gather. A galaxy cluster is the largest kind of object held together by its own gravity — a dense swarm of hundreds or even thousands of galaxies, all orbiting a common center, bound together in a single colossal system that can span millions of light-years.

But here is the surprise: the galaxies you can see are only a small fraction of a cluster's mass. Most of the ordinary matter is not in galaxies at all but in a tenuous, extremely hot gas filling the space between them — the intracluster medium, heated to tens of millions of degrees so that it glows in X-rays. And even all that gas plus all the stars together make up only about a sixth of the total; the rest, roughly 85 percent, is dark matter. In fact it was a galaxy cluster (Coma) where Fritz Zwicky first inferred dark matter in the 1930s, noticing the galaxies moved far too fast to stay bound by their visible mass alone. Smaller, looser gatherings of a few to a few dozen galaxies are called groups; our Milky Way belongs to one, the Local Group.

Clusters matter as the universe's largest gravitationally bound laboratories and as the nodes of the cosmic web, sitting where its filaments cross. They are powerful cosmic scales — astronomers weigh them three independent ways (galaxy speeds via the virial theorem, X-ray gas, and gravitational lensing of background galaxies) and the answers agree on lots of dark matter. The crowded cluster environment also transforms galaxies, stripping their gas and quenching them, which is why cluster cores are dominated by red ellipticals and lenticulars.