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A galaxy is a huge gravitationally bound system of stars, interstellar gas and dust, plasma, and (possibly) unseen dark matter. Typical galaxies contain ten million to one trillion (107 to 1012) stars, all orbiting a common center of gravity. In addition to single stars and a tenuous interstellar medium, most galaxies contain a large number of multiple star systems and star clusters as well as various types of nebulae. Most galaxies are several thousand to several hundred thousand light years in diameter and are usually separated from one another by distances on the order of millions of light years.
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 some evidence that supermassive black holes may exist at the center of many, if not all, galaxies.
Intergalactic space, the space between galaxies, is filled with a tenuous plasma with an average density less than one atom per cubic meter. There are probably more than a hundred billion (1011) galaxies in our observable universe.
The method of galactic formation is a major open question in astronomy. Theories may be divided into two categories: top-down and bottom-up. In top-down theories such as the Eggen–Lynden-Bell–Sandage (ELS) model, protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years.[2] In bottom-up theories such as the Searle-Zinn (SZ) model, globular clusters form first, and then a number of such bodies accrete to form a larger galaxy. Modern theories must be modified to account for the probable presence of large dark matter halos. A sketch of a galactic formation model follows.
Shortly after recombination, baryonic matter begins to condense around cold dark matter halos. Zero-metal high-velocity halo stars (called Population III stars) are the first to develop around a protogalaxy as it starts to contract. These huge stars quickly supernova, releasing heavy elements into the interstellar medium. Within the next billion years, globular clusters, the central supermassive black hole and galactic bulge of metal-poor Population II stars form. Within two billion years, the remaining material settles into a galactic disk. The galaxy will continue to absorb infalling material from high velocity clouds and dwarf galaxies throughout its life; the cycle of stellar birth and death will increase the abundance of heavy elements, eventually allowing the formation of planets.