A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas and dust, and dark matter.
The word galaxy is derived from the Greek galaxias, literally "milky", a reference to the Milky Way. Examples of galaxies range from dwarfs with just a few thousand (103) stars to giants with one hundred trillion (1014) stars, each orbiting their galaxy's own center of mass. Galaxies can be categorized according to their visual morphology, including elliptical, spiral, and irregular. Many galaxies are believed to have black holes at an active center. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times that of our Sun. As of May 2015, EGS-zs8-1 is the most distant known galaxy, estimated to be 13.1 billion light-years away and to have 15% of the mass of the Milky Way.
There are approximately 170 billion (1.7 × 1011) galaxies in the observable universe. Most of the galaxies are 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs). The space between galaxies is filled with a tenuous gas with an average density less than one atom per cubic meter. The majority of galaxies are gravitationally organized into associations known as galaxy groups, clusters, and superclusters. At the largest scale, these associations are generally arranged into sheets and filaments that are surrounded by immense voids.