A galaxy is a large system of stars, interstellar gas, dust, dark matter and possibly dark energy, bound by the forces of gravitational interaction. The number of stars and sizes of galaxies can be different. As a rule, galaxies contain from several million to several trillions (1,000,000,000,000) stars. In addition to ordinary stars and the interstellar medium, galaxies also contain various nebulae. Dimensions of galaxies from several thousand to several hundred thousand light years. And the distance between galaxies reaches millions of light years.