Such galaxies have probably 'eaten' several nearby galaxies, and lends support to the idea that mergers can make ellipticals. Dark Matter Many spirals are observed to have flat rotation curves. Many galaxies are also thought to harbor central super-massive black holes, with masses between 1 million and 1 billion solar masses. The rotation curve near the center of the Andromeda galaxy, showing high speeds at small distances as indirect evidence for a black hole It appears that ellipticals also possess substantial amounts of dark matter. Galaxy Clusters Stars come in clusters and so do galaxies.
Groups contain a few big galaxies and several smaller ones.
Clusters contain of order 1000 galaxies. Apparently even clusters possess dark matter, meaning in excess of what each individual galaxy has on its own.