Dark matter halo growth refers to the gradual buildup of invisible dark matter around galaxies over cosmic time, forming massive halos that act as gravitational anchors. These halos are essential to the formation and evolution of galaxies because they provide the gravitational framework within which galaxies form, merge, and grow. Dark matter does not emit or absorb light, but its presence is inferred from its gravitational effects on visible matter, such as stars and gas, as well as on galaxy rotation curves and gravitational lensing.
The growth of dark matter halos begins in the early universe with tiny fluctuations in matter density shortly after the Big Bang. Regions with slightly higher density attracted more matter through gravity, forming small dark matter clumps. Over billions of years, these clumps merged to form larger and larger halos in a process known as hierarchical structure formation. Galaxies formed within these halos when gas cooled and collected in their centers, eventually giving rise to stars and galactic disks.
As time passes, dark matter halos continue to grow through two main mechanisms: accretion and mergers. Accretion occurs when a halo gradually pulls in surrounding dark matter from intergalactic space. Mergers happen when two halos collide and combine, often bringing their galaxies together and triggering galaxy interactions or mergers. The size and mass of a dark matter halo determine the type of galaxy that forms within it, as well as how that galaxy evolves over time.
Dark matter halo growth also affects the motion of stars, the distribution of satellite galaxies, and the ability of a galaxy to gather gas for star formation. Larger halos can retain gas more effectively, supporting long-term star formation, while smaller halos may lose gas due to supernova explosions or interactions with larger galaxies. Understanding dark matter halo growth is crucial for explaining the large-scale structure of the universe and the cosmic web, as well as the formation histories of galaxies like the Milky Way.