A galaxy is a gravitational system of stars, interstellar gas, stellar fragments, dust, and dark matter. The word galaxy is originated from the Greek word galaxies (γαλαξίας), literally meaning 'milky', a reference to the Milky Way. Galaxies are in size from small with just a few hundred million (108) stars to colossi with one hundred trillion (1014) stars, each orbiting its galaxy's center of mass.Galaxies are characterized according to their visual morphology as oval, spiral, or irregular. Many galaxies are believed to have supermassive black holes at their cores. The Milky Way's central black hole, known as Sagittarius A*, has a weight of four million times greater than the Sun. Since April 2016, GN-z11 is the oldest and best reserved observed galaxy with a comoving distance of 32 billion light-years from Earth and observed as it existed just 400 million years after the Big Bang.The space between galaxies is filled with an unsubstantiated gas (the intergalactic medium) having an average mass of less than one atom per cubic meter. Most galaxies are gravitationally systematized into groups, clusters, and superclusters. The Milky Way is part of the Local Group, which is ruled by it and the Andromeda Galaxy and is part of the Virgo Supercluster. At the biggest scale, these associations are mostly arranged into sheets and filaments surrounded by immense spaces. The biggest structure of galaxies yet to recognize as a cluster of superclusters that has been termed Laniakea, which holds the Virgo supercluster.