Galaxy

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Stars are almost always found in collections called galaxies, together with gas, dust, and large amounts of dark matter detected by its gravitational effects. These are all held together by gravitational attraction and orbit a common centre. There is some evidence that black holes may exist at the centre of some or most galaxies. Galaxies come in three main types: ellipticals, spirals, and irregulars. Our own galaxy, the Milky Way, sometimes simply called the Galaxy (with uppercase), is a large spiral about 10 kpc in radius, and contains about a trillion times the mass of the sun.

Most galaxies do not exist by themselves. Our galaxy is a member of the Local Group, and together with the Andromeda Galaxy dominates it; overall the group contains about 30 galaxies in a space about 1 Mpc across. Larger structures are called clusters, and may contain thousands of galaxies packed into a few Mpc. Superclusters are giant collections containing tens of thousands of galaxies, found in clusters, groups and sometimes individually; as far as we can tell the universe is uniform at scales above this. The space between galaxies is relatively empty.