Astronomers have observed three types of black holes in the universe. Stellar-mass black holes formed from the collapse of a massive star, intermediate mass black holes found in some star clusters, and supermassive black holes that lurk in the centers of galaxies.

If microscopic black holes born a fraction of a second after the Big Bang exist, then at least one may fly through the solar system per decade, generating tiny gravitational distortions that scientists can detect.