[GW150914]

The whole point of a black hole is that nothing can support its own mass past a certain point. A black hole is a region more than it is an object, with a singularity beyond an event horizon. If there really is a singularity, it is a point or ring, not a sphere. There are other conjectures which replace the singularity with something else, like masses of strings, but it doesn’t really matter to the universe outside of the event horizon.

As to it being a source of dark matter, the idea that black holes and other compact objects are dark matter is encapsulated in the MACHO (MAssive Compact Halo Object) theory. It is extremely unlikely to be correct, because observations largely rule this out. Black holes warp spacetime around them and result in detectable phenomena as a result of gravitational lensing. It would appear that such lensing is not common enough to allow for black holes in sufficient quantity to explain dark matter.