A cD galaxy is a type of giant elliptical galaxy found at the centers of galaxy clusters. These galaxies are characterized by their large size, bright luminosity, and extensive halo of stars. They often contain a significant amount of dark matter and can have a complex structure, including multiple stellar populations. cD galaxies typically form through the merging of smaller galaxies and can accumulate mass over time by attracting nearby galaxies. They are often surrounded by a large number of satellite galaxies and are important for studying the evolution of galaxy clusters and the large-scale structure of the universe, including the role of dark matter.