Galaxies are massive structures within the universe that contain billions of stars, gas, dust, and dark matter. They vary in size from a few thousand stars to trillions of stars in the largest galaxies. The most common types of galaxies are spiral, elliptical, and irregular. Spiral galaxies have a flat, rotating disk with a central bulge and spiral arms emanating from the center. Elliptical galaxies are typically round or oval-shaped and have less organized structure. Irregular galaxies have a chaotic mix of shapes and sizes.

The theory of galaxy formation suggests that they were created from gravitational collapse of clouds of gas and dust in the early universe. Over time, as the gas and dust collapsed, they became denser and hotter, eventually forming the first stars and galaxies. Today, galaxies continue to evolve and change through various processes like mergers with other galaxies and interactions with intergalactic gas.