cD galaxies are a type of giant elliptical galaxy found in the centers of galaxy clusters. They are characterized by their large size, extended halos of stars, and often contain a significant amount of dark matter. cD galaxies typically have a bright core and can host many smaller satellite galaxies within their gravitational influence. These galaxies are formed through the merging of smaller galaxies and the accumulation of material from their surroundings. The "cD" designation stands for "central dominant," indicating their position as the most massive galaxy in a cluster. Examples of cD galaxies include M87 and NGC 4874.