What are galactic clusters?

A galaxy cluster is any group of galaxies whose gravity is enough to overcome the general expansion of the universe and hold it together.

About half of all galaxies are found in clusters of one sort or another. The least impressive clusters, known as galaxy groups, typically contain a handful of large galaxy systems surrounded by a few dozen smaller dwarf galaxies – our Milky Way Galaxy, for example, is one of three large spirals in the so-called Local Group.

Dense clusters, however, may contain up to a couple of thousand galaxies including dozens of large spirals and elliptical (ball-shaped) galaxies.

Curiously, the size of the clusters does not vary all that much – they are always around one to ten megaparsecs (or 3-33million light years) across.