Our Galaxy

A galaxy is a massive gravitationally bound system of stars, an interstellar medium of gas and dust, and an unknown dark matter. Typical galaxies contain ten million[1] to one trillion[2] (107 to 1012) stars, all orbiting a common center of gravity. Galaxies can also contain a large number of multiple star systems and star clusters as well as various types of interstellar clouds. Most galaxies are a thousand to a hundred thousand[2] parsecs in diameter and are usually separated from one another by distances on the order of millions of parsecs (or megaparsecs).[3]
Although theoretical, dark matter appears to account for around 90% of the mass of most galaxies—the nature of these unseen components is not well understood. There is also some evidence that supermassive black holes may exist at the center of many, if not all, galaxies. The Milky Way galaxy appears to harbor just such an object within the core region.[4]
Intergalactic space, the space between galaxies, is filled with a tenuous gas with an average density less than one atom per cubic metre. There are probably more than a hundred billion (1011) galaxies in our observable universe.[5]http://en.wikipedia.org/wiki/Galaxy