Ultralight is a contributor to the energy density of a universe that dilutes faster than radiation as the universe that it is in expands. The mechanism for this depends on the exact type of ultralight. Because its energy density dilutes quickly, it has a higher pressure than normal matter and radiation, and hence gravitationally attracts more strongly per unit energy. Due to its rapid rate of rarefaction, when ultralight is present it is more likely to dominate small and young universes.