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Galaxy formation is currently thought to occur with the collapse of gas clouds at a high-density region of dark matter. Simulations, as well as galaxy morphology population statistics are used to test theories. Theories have been developed regarding formation of a galaxy (e.g., spiral galaxy) via a single collapse (a monolithic collapse, for the monolithic collapse model aka top down model) or to build galaxies from smaller collapses, e.g., out of stellar clusters and galaxy mergers (the bottom up model). Current thought (represented by the Lambda-CDM model) is closer to the latter, and the initial smaller collapses occur in regions with sufficient gas and dark matter.
A (hydrogen) gas cloud drawn into a region of dark matter (dark matter halo) must cool sufficiently to allow star formation, which requires sufficient transparency to allow electromagnetic radiation (EMR) to carry away energy. This cooling ability depends on the state of the hydrogen, the most efficient state being neutral atomic hydrogen (HI). The mass of the dark matter halo affects this cooling ability, with galaxies apparently forming stars within a limited mass range that tops out at about 1012 solar masses. A lower bound is evident as well.