Galaxies are composed of gas, dust, dark matter and a multitude of stars and solar systems, all held together by gravity. Galaxies vary greatly in size, containing approximately 10 million to 10 trillion star systems. The Milky Way galaxy, of which Earth's solar system is a part, contains about 200 billion stars.
Galaxies have various shapes. More than half the galaxies in the universe are elliptical, which means they are slightly rounded and without a spiral pattern. Elliptical galaxies have no gas or dust. Some are only 1 percent of the size of the Milky Way, while others are five times as large. Spiral galaxies, including the Milky Way, are disc-shaped. Astronomers have concluded that the center of the Milky Way is a massive black hole based on the energy expelled and the immense amount of gravity needed to account for the movement of the stars. There is enough excess dust and gas in the Milky Way galaxy to create billions more stars.
Galaxies are not randomly scattered throughout the universe, but rather bound together by gravity into galactic clusters. Some rich galactic clusters contain more than 2,500 galaxies, while other clusters contain 100 galaxies or less. The cluster of which the Milky Way galaxy is a part contains only about 50 galaxies. Galactic clusters are clumped together into long filaments known as walls with spaces between called voids, giving the universe roughly the structure of a spider web.