Supermassive black holes are those back holes whose mass in more tan 10,00,000 times the mass of the Sun. Such black holes can be found at the centre of many galaxies. Supermassive back holes are thought to evolve when many smaller back holes merge at the centre of a glazy, and start swallowing everything that comes their way.
Such a black hole is a remnant of an exploded sun much bigger than our own. The explosion is a rare phenomenon called supernova, which happens when these developed suns use up all their nuclear fuel.
Without fuel to maintain the huge pressure required to counter gravity the star first implodes, and then the outer layers rebound against the sun's core and are violently ejected into space, in a process that is one of the most powerful explosion that occur in nature. Simultaneously the massive core continues to cave in. It rapidly collapses into itself and forms a black hole.