Pictures of galaxy clusters and galaxy groups
A guide to galaxy clusters and groups, and images of these gigantic cosmic structures.
Galaxy clusters are collections of gravitationally-bound galaxies and are the largest known structures held together by gravity in the Universe.
Galaxy groups are similarly gigantic, but they generally contain fewer galaxies and are not as large as galaxy clusters.
Galaxy clusters can contain up to thousands of galaxies and are fascinating objects for astronomers and cosmologists to observe because studying them reveals information about the hidden secrets of the Universe.
They are vital targets for understanding the properties of dark matter, for example.
Dark matter is the name given to the mysterious, invisible substance that accounts for about 85% of all matter in the Universe.
It cannot be directly observed, but astronomers can infer its existence because of its gravitational influence on visible matter.
Dark matter accounts for 85% of the total mass of a galaxy cluster: the galaxies themselves make up just 2% of the total mass.
In the case of galaxy clusters, the total visible mass is not enough to account for the gravity required to hold them together, given the speed at which they travel.
Extra mass in the form of hot gas known as the intracluster medium does account for some of the extra gravity required to hold the galaxy clusters together, but it's still not enough.
The extra missing material is known as dark matter.
Famous galaxy clusters beloved by astronomers and astrophotographers include the Virgo Cluster, the Fornax Cluster and the Hercules Cluster.
Galaxy groups are much smaller than galaxy clusters, but there is no clear dividing line between the two. Galaxy groups generally contain up to 50 galaxies gravitationally bound.
Below is a selection of images of galaxy clusters captured by BBC Sky at Night Magazine.