What are the differences between normal black holes and supermassive black holes?
Black holes are predicted by the Schwarzschild solution to the General Theory of Relativity. Black holes have not been observed directly. However, objects have been observed indirectly which are so small and massive they can only be black holes. There are thought to be three types of black hole.
Primordial black holes are thought to have been created by the high temperatures and pressures which existed after the big bang. There are theories that dark matter consists of primordial black holes.
Stellar black holes are formed when the core of a star with a mass of more than three times that of the Sun collapses under gravity. They have a radius of up to about 30km.
Supermassive black holes are millions or billions of times the mass of the Sun. It is thought that most large galaxies have a supermassive black hole at their centres. Our Milky Way galaxy has a black hole at its centre with a mass of 4 million times that of the Sun. It is less than the size of the orbit of Neptune.