A galaxy cluster is a large group of galaxies that are held together by gravity. These clusters can contain hundreds or even thousands of galaxies, along with hot gas and dark matter. They are among the largest structures in the universe and can span millions of light-years across. Galaxy clusters are important for studying the universe because they help astronomers understand how galaxies form and evolve. They also provide insights into the distribution of dark matter, which makes up a significant portion of the universe's mass but cannot be seen directly.