Clusters of galaxies containing hundreds or even thousands of individual galaxies are held together by gravity. As MIT highlights, they move through a sea of hot gas called the intracluster medium, and give off X-ray radiation that we can see using space-based telescopes.
https://www.techspot.com/news/89088-astronomers-discover-new-galaxy-clusters-hiding-right-front.html?utm_source=dlvr.it&utm_medium=blogger
https://www.techspot.com/news/89088-astronomers-discover-new-galaxy-clusters-hiding-right-front.html?utm_source=dlvr.it&utm_medium=blogger