What are galactic clusters?
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.