Can there be such a thing as a black hole cluster, with multiple black holes in very very small distances?
I have never heard about the existence of black hole clusters. There are certainly super-massive black holes, but black holes are generally found at the centers of galaxies and certain star clusters. Indeed, because of their intense ability to warp space/time around them, stars and other matter accumulate and swirl around black holes. And while there are clusters of galaxies, there are no clusters of black holes alone... at least none have been found as yet. (And one would think that as soon as black holes got close enough to cluster, they would swallow each other up.) By the way, it is believed that the center of our galaxy, the Milky Way, contains a black hole.
And just as a matter of correction, sanjiwan, there are no black holes known to exist in our solar system. If any did exist, we probably wouldn't.
Yes, there are so many black holes in solar system. Black holes are the area in universe having excessive gravity in them . The gravity is so strong that whatever comes within their area of gravity they swollow them. The scientists believe that so many kinds of stars were swolled by the black holes over the years.