Most galaxies are members of a cluster, a grouping of several to thousands of galaxies. Our Milky Way, for example, is a member of the "Local Group," a set of about fifty galaxies whose other large member is the Andromeda galaxy about 2.3 million light-years away. The closest large cluster of galaxies to us is the Virgo Cluster, with about 2000 members; its center is about 50 million light-years away. The clustering of galaxies influences how any particular member galaxy will evolve, but what happens and how it happens are not well understood. The cluster's influences on the star-formation activity within its galaxies is a particularly interesting question because the star formation rate helps set the luminosity of a galaxy, its supernovae activity, and the processing of its hydrogen gas into heavier elements.
"Neutral Hydrogen Gas, Past and Future Star Formation in Galaxies in and around the 'Sausage' Merging Galaxy Cluster," Andra Stroe, Tom Oosterloo, Huub J. A. Rottgering, David Sobral, Reinout van Weeren, and William Dawson, MNRAS 452, 2731, 2015.