Galaxies grow primarily through hierarchical merging, where smaller galaxies and gas clouds coalesce to form larger structures.

Galaxy formation is not a monolithic process but rather a 'bottom-up' scenario known as hierarchical merging. This model suggests that the first structures to form were relatively small, which then merged repeatedly over cosmic time to build up the massive galaxies we see today. Evidence for this comes from observations of galaxy interactions and mergers throughout the universe's history, particularly in galaxy clusters, and from computational simulations that accurately reproduce the observed galaxy population. These mergers can dramatically alter galactic morphology, triggering bursts of star formation and feeding central supermassive black holes.