One theory is that they could be gateways to other galaxies. This however is probably only science fiction.
What do we Know about Black Holes?
Though the term 'black hole' was only coined in 1967, the study of objects that are so large that even light cannot escape, goes back to the late 18th century. It was the development of Einstein's General Relativity that helped define the properties of Black Holes.
Black holes are thought to be objects of large density who's gravitational pull is so strong that even light cannot escape it. Because light cannot escape they are hard to detect.
Supermassive Black Holes
What is a Supermassive Black Hole?
This type of Black Hole is thought to be the largest kind. They are thought to exist at the centre or most if not all galaxies including our own Milky Way.
How do they form?
They could form from smaller Black Holes that started out as a star that went nova as Black Holes grow as they take in more matter.
They could have been formed at the time of the big bang and as such could be what is known as a Primordial Black Hole. This idea fits with them being at the centre of a Galaxy as it could be the Black Holes Gravity that turns the Galaxy and moves it around the Universe.
Extract from my Hub