A PBH is a hypothetical type of black hole that is formed not by the gravitational collapse of a large star but by the extreme density of matter present during the universe's early expansion. It has been proposed that dark mater is made up of primordial black holes. One theory proposes that they are in the mass range of 1017 g to 1026 g, based on the expectation that at this low mass they would behave as expected of other particle candidates for dark matter. They would be within the typical mass range of asteroids. Another theory is that dark matter consists of larger black holes, about 30 times the mass of the sun (about 6 × 1034 g). One way to detect primordial black holes is by Hawking radiation, if they are small enough to radiate, and if Hawking radiation exists, otherwise they will be extremely difficult to detect.