In space, the gravitational pull of massive objects is irresistible to smaller ones. Moons are locked in orbit around planets. Planets, asteroids and comets orbit more massive stars, and stars collect around supermassive black holes, forming galaxies.
Large galaxies, like the Milky Way, attract smaller galaxies. Our solar system's cosmic neighborhood spans 100,000 light-years and contains between 100 billion and 400 billion stars. The Milky Way is so big that, over billions of years, its mass has captured numerous dwarf galaxies, which contain no more than a few billion stars, as satellites.
