The universe is littered with galaxy clusters – huge structures piled up at the intersections of the cosmic web. A single cluster can span millions of light-years across and be made up of hundreds, or even thousands, of galaxies.
However, these galaxies represent only a few percent of a cluster’s total mass. About 80% of it is dark matter, and the rest is a hot plasma “soup”: gas heated to above 10,000,000℃ and interwoven with weak magnetic fields.
