It is believed that there are an extremely large number of black holes in the universe. Black holes can form from the collapse of stars, and it is thought that there are at least tens of millions of black holes in the Milky Way galaxy alone. However, it is difficult to accurately estimate the total number of black holes because they do not emit light and therefore cannot be directly observed. In addition, black holes can be very small, with masses only a few times that of the Sun, or they can be extremely massive, with masses millions of times that of the Sun.