A galaxy is a vast system that contains stars, planets, gas, dust, and dark matter, all bound together by gravity. The Milky Way, for example, is the galaxy that contains our solar system. Galaxies can vary in size and shape, with some being spiral, elliptical, or irregular. There are billions of galaxies in the universe, each with millions or even billions of stars. The study of galaxies helps astronomers understand the formation and evolution of the universe. Tools like the Hubble Space Telescope have allowed scientists to observe distant galaxies and learn more about their properties and behaviors.