A galaxy is a massive system that consists of stars, stellar remnants, interstellar gas, dust, and dark matter, all bound together by gravity. The Milky Way, which is the galaxy that contains our Solar System, is just one of billions of galaxies in the universe. Galaxies can vary in size, shape, and composition, and they are classified into different types, such as spiral, elliptical, and irregular. Galaxies are fundamental to our understanding of the universe. They are the building blocks of cosmic structure and play a crucial role in the formation of stars and planetary systems. The study of galaxies helps astronomers learn about the history and evolution of the universe, including phenomena like dark matter and black holes.