A group of galaxies is known as a galaxy cluster. These clusters are massive structures that can contain dozens to thousands of galaxies, held together by gravity. Galaxy clusters vary in size and can span millions of light-years across. Clusters are important in cosmology as they provide insights into the distribution of matter in the universe and serve as a framework for understanding large-scale structures. The Milky Way, for instance, is part of the Local Group, a small cluster of galaxies. Research on galaxy clusters helps scientists study dark matter, dark energy, and the evolution of galaxies.