A cancer cluster of galaxies refers to a group of galaxies that are located close to each other in space and are part of a larger cosmic structure. These clusters can contain hundreds or thousands of galaxies, along with hot gas and dark matter. They are important for studying the formation and evolution of galaxies and the universe. The term "cancer" in this context is not related to the disease but is derived from the Latin word for crab, which is associated with the constellation Cancer. Clusters like the Virgo Cluster and Coma Cluster are examples of such structures, providing valuable insights into cosmic phenomena.