Clusters of galaxies are gigantic structures of the Universe. Galaxies emit a lot of gravity. This causes nearby galaxies to attract each other and group together to form clusters.
Our galaxy, the Milky Way, is part of a small cluster called Local Group.
Within a cluster, galaxies revolve around each other, and they even frequently collide. The size and mass of a cluster varies according to the galaxies that form it, but the distance between one end and the other is always several million light years.
The clusters are not only composed of galaxies, but also of large clouds of hot gas. In general, they are remnants of galaxies that die when they collide with each other. But most of the mass of the cluster is dark matter. It is believed that each cluster can have up to five times more dark matter than visible matter.
The clusters of galaxies have a spherical or spiral shape and revolve around a central nucleus. This nucleus hosts most of the hot gas and emits a large amount of X-rays. The densest galaxies are located near the center, where gravity is greatest. Around, thousands of galaxies are dispersed among clouds of gas.
Superclusters of galaxies
Superclusters of galaxies are the second largest structure in the Universe. Only the huge walls of superclusters surpass them. As the name implies, superclusters are clusters of galaxy clusters and are found throughout the known Universe.
Clusters of galaxies join at their ends, and form huge chains. The gravity in the superclusters is so great that it even slows the expansion of the Universe. All matter is attracted, and that is why huge empty spaces are created between some superclusters and others.
Our Local Group is part of the Virgo Supercluster which, in turn, is integrated into a superstructure called Laniakea ("immense sky", in Hawaiian).
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