This spectacular view from the NASA/ESA Hubble Space Telescope peers into the distant Universe to reveal a massive galaxy cluster called PLCK G308.3-20.2.

Galaxy clusters such as PLCK G308.3-20.2 contain thousands of galaxies of all ages, shapes and sizes, together totaling a mass thousands of times greater than that of our Milky Way Galaxy.

At one point in time they were believed to be the largest structures in the Universe — until they were usurped in the 1980s by the discovery of superclusters, which typically contain dozens of galaxy clusters and groups and span hundreds of millions of light-years.

However, clusters do have one thing to cling on to; superclusters are not held together by gravity, so galaxy clusters still retain the title of the biggest structures in the Universe bound by gravity.

One of the most interesting features of galaxy clusters is the stuff that permeates the space between the constituent galaxies — the intracluster medium.

High temperatures are created in these spaces by smaller structures forming within the cluster. This results in the intracluster medium being made up of plasma — ordinary matter in a superheated state.

Most luminous matter in the cluster resides in the intracluster medium, which is very luminous X-rays.

However, the majority of the mass in a galaxy cluster exists in the form of non-luminous dark matter.

Unlike plasma, dark matter is not made from ordinary matter such as protons, neutrons and electrons.

It is a hypothesized substance thought to make up 80 % of the Universe’s mass, yet it has never been directly observed.

This image of PLCK G308.3-20.2 was taken by Hubble’s Advanced Camera for Surveys (ACS) and Wide-Field Camera 3 (WFC3) instruments as part of an extensive observing program called Reionization Lensing Cluster Survey (RELICS).

RELICS imaged 41 massive galaxy clusters over the course of 390 Hubble orbits and 100 Spitzer Space Telescope observing hours, aiming to find the brightest distant galaxies.

Studying these galaxies in more detail with both current telescopes and the future NASA/ESA/CSA James Webb Space Telescope will hopefully tell us more about our cosmic origins.