Our mathematical understanding of physics "breaks down" at the singularity of the black hole; in a very real sense, we CANNOT KNOW what happens in there. Check back in about 50 years; we may have a better understanding of physics by then.
But until then, I think the answer would be "Probably."
Yes. They have a density of infinity
In theory it is compressed down into a singularity. This is why black holes are the densest objects in the universe. All that mass is being squeezed down into a single point in space.
Probably the supermassive black holes that are believed to be at the centers of many (perhaps most) large galaxies. These may be related to quasars.
Black Holes.
Answer
No, dark matter is quite a different kind of thing. A dark hole may have absorbed some dark matter, but pressumably that would become indistinguishable from the normal matter, once it gets crushed by the enormous gravity of the black hole.
There are already black holes within the universe
Even though black holes suck through parts of the universe, the universe is inevitably big, and growing so as the universe is being sucked into another dimension by black holes, it is also expanding.
Black Holes' can 'eat' any type of matter in the Universe.
Probably stellar mass black holes
No.
Yes.