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.
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