A gravitationally bound collection of hundreds to thousands of galaxies, along with hot gas and dark matter. These are among the largest known structures in the universe, held together by their mutual gravitational attraction.
From Greek 'galaxias' (milky circle) and Latin 'clustrum' (a bunch or group). The concept developed in the 1930s when astronomers like Fritz Zwicky first recognized that galaxies weren't randomly distributed but grouped together in massive collections.
Galaxy clusters are like cosmic cities where galaxies can actually collide and merge, but the space between stars is so vast that individual stars rarely hit each other - it's like two swarms of bees passing through each other! The hot gas between galaxies in a cluster can reach temperatures of 100 million degrees, making it glow in X-rays.
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