Galactic clusters are large groups of galaxies that are bound together by gravity. They can contain anywhere from a few dozen to thousands of galaxies, along with gas and dark matter. These clusters are the largest known structures in the universe and can span millions of light-years across. Within a galactic cluster, galaxies can interact with each other, leading to phenomena such as galaxy mergers and the formation of new stars. The study of galactic clusters helps astronomers understand the distribution of matter in the universe and the evolution of galaxies over time.