Galaxy clusters are the largest gravitationally bound structures observed in the Universe today, and whilst at first sight (in the literal sense) they may appear to simply be a collection of galaxies, they are much more than the sum of their parts.
Their evolution is primarily driven by the dominant matter component of the Universe, the so-called Dark Matter that makes up 25% of the Universe’s energy budget. This condensed into high density regions in the early Universe, forming the gravitational potential wells that baryonic matter fell into.
Galaxy clusters have masses in the range of 1014 to 1015 times the mass of our Sun, with up to thousands of galaxies. Whilst the dark matter accounts for 90% of a cluster’s mass, the main standard model component in galaxy clusters is the hot (highly ionised) gas which fills the intra-cluster medium (the space between the galaxies).
This gas is mostly hydrogen formed from the big bang, and is the dominant mass component (9%) over the galaxies themselves (1%), and has temperatures of the order of 10–100 million Kelvin, and densities around 10−3 electrons cm−3.
This is is the Perseus cluster. The X-Ray image on the left shows the gas distribution (mostly hydrogen), and the optical image on the right shows the galaxy distribution. The monster looking galaxy at the centre hosts an active galactic nuclei, and the red filaments show hydrogen gas being accreted. Images and further details available from Fabian et al. 2011 and Sanders et al 2016.
This movie is a simulation showing how one cluster passes through another, disturbing and mixing up the gas. Note how remarkably similar the image is to the real X-Ray image:
A more nerdy insight:
The physics of galaxy clusters can be broken down into thermal, and non-thermal components, where in both cases it is the electrons that are the primary particle that radiates energy. The thermal emission originates from electrons gaining kinetic energy from falling into a large gravitational potential well, and re-radiating it as X-rays (Bremsstrahlung radiation). Thus, observing the diffuse X-ray emission from clusters directly maps the temperature and density profiles of the ICM. The non-thermal emission originates from the highest energy ultra-relativistic electrons that have been accelerated from radio loud Active Galactic Nuclei (AGN) and galaxy cluster merger events, and re-radiate their energy as radio waves (synchrotron radiation). At first sight, observing synchrotron radiation pinpoints radio loud AGN present in the cluster, but in recent years it has become apparent that some clusters host diffuse synchrotron radiation throughout the ICM, and others host arc-like structures seemingly unassociated with any AGN. Thus, the non-thermal component in galaxy clusters is becoming increasingly studied in order to understand its impact on the energy and pressure budget of the ICM. In particular, a consequence of observing diffuse synchrotron radiation throughout the ICM (amongst other evidence) shows that magnetic fields are present throughout galaxy clusters. The origin of these large-scale magnetic fields is an open question going back to the primordial quantum fluctuations in the first second of the big bang, and with galaxy clusters being giant plasma physics laboratories, it has recently become apparent that magnetic fields play a vital role in understanding the evolution of a galaxy cluster.