In a galaxy cluster, the galaxies themselves are the smallest share of the mass
A galaxy cluster looks like a swarm of galaxies bound together, yet the galaxies move too fast for their own gravity to hold them. Scorching gas between them outweighs the galaxies about twice over, and even that falls short. Most of a cluster's mass is dark matter nobody has identified.
Groups and clusters are the biggest objects gravity has bound together so far, and they make up the densest parts of the universe's large-scale web. In cold dark matter models small structures collapse first and build up into larger ones, so clusters are latecomers, assembled within the last 10 billion years. They can contain anywhere from about ten to thousands of galaxies.
Groups are the smaller kind, typically holding up to 50 galaxies across 1 to 2 megaparsecs, with a combined mass near ten trillion suns and member galaxies moving at around 150 kilometres per second relative to one another. The Milky Way belongs to the Local Group, which has more than 54 members. A 2017 study by S. Paul, R. S. John and colleagues proposed drawing the line using scaling laws: anything lighter than about 80 trillion solar masses counts as a group.
Clusters have no sharp boundary with groups. X-ray telescopes reveal that the space inside them is filled with the intracluster medium, gas at tens of millions of kelvin. Because that gas sits roughly balanced against the cluster's gravity, its temperature reveals the total mass, which comes out about six times what galaxies and gas together provide. In a typical cluster perhaps only 5 percent of the mass is galaxies and around 10 percent hot gas.
Astronomers find clusters in several ways. Optical and infrared surveys look for crowds of galaxies at the same redshift, while X-ray surveys spot the glowing gas; clusters rank with active galactic nuclei as the brightest X-ray sources beyond our galaxy. Hot electrons also scatter light from the cosmic microwave background, leaving a faint shadow at some radio frequencies, and a cluster's gravity bends the images of galaxies behind it, letting researchers map its invisible matter.
Source: Galaxy groups and clusters
