Black holes are usually formed in the explosive deaths of massive stars. In dense environments like globular clusters and the cores of massive galaxies, several black holes can merge together, forming an even more massive black hole. In addition, black holes at the centers of galaxies can accrete infalling gas, gaining in mass and size. If this process continues for billions of years, the black hole can become a supermassive black hole, like the one located at the center of our Milky Way Galaxy, weighing in at four million times the mass of the sun.
An international team of researchers have considered the late Stephen Hawking’s 1974 theory of the existence of primordial black holes, born shortly after the Big Bang, and his speculation that they could make up a large fraction of the elusive dark matter scientists are trying to discover today.