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.

In any globular cluster, all its stars formed at the same time, from the same cloud of gas. The Milky Way has around 150 known globular clusters; these objects are excellent tools for studying, for example, the history of the Universe, or the dark matter content of the galaxies they orbit.

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.