Clusters of galaxies are the largest structures in the Universe, which are in equilibrium. They consist roughly of 80% dark matter, 15% hot gas, and finally a few percent stars and cold gas and dust.
We observe clusters in several ways, such as through the X-ray emitting gas, by using gravitational lensing of background galaxies, and through the upscattered photons from the cosmic microwave background (the Sunyaev-Zeldovich effect).
We use clusters to learn about the radial distribution of both the hot gas and the mass dominating dark matter. Observations tell us for instance about the radial distribution of the gas temperature or the radial distribution of the dark matter density. These observed profiles can then be compared with the results of numerical simulations or theoretical predictions to investigate the properties of dark matter. Finally, we study the ongoing collision between two galaxy clusters, which provides one of the strongest confirmations of the presence of dark matter.