Supermassive black holes are found at the centres of galaxies. They have masses around a 100 million times the mass of the Sun and it is currently unknown how these objects form.
A Survey of Quasar Host Galaxies
Source: John Bahcall (Institute for Advanced Study, Princeton), Mike Disney (University of Wales), and NASA
In the 1960's it was found that some galaxies, called active galaxies, have bright nuclei which cannot be explained by starlight. Much evidence now suggests that active galaxies are powered by supermassive black holes.
The brightest of the active galaxies are quasars, which are more luminous than the entire Milky Way Galaxy but are no bigger than our Solar System. They appear as bright stars and were overlooked by astronomers for many years. It is now known that quasars are the bright central part of very distant galaxies.
It was originally thought that only active galaxies could contain supermassive black holes, but there is growing evidence that even 'quiet' galaxies, like our own Milky Way Galaxy, may have a supermassive black hole at its centre.
In the case of the Milky Way Galaxy there is no bright emission coming from the Galaxy's centre as occurs in active galaxies. This suggests that there is currently no material circling the event horizon of the black hole, which is where such emission arises. Instead, the black hole has been revealed by studying the motion of stars within the central region of the Galaxy. The suspected black hole is estimated to have a mass equivalent to 2 million Suns.