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.
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.
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.
There are already black holes within the universe
Black Holes' can 'eat' any type of matter in the Universe.
The density of a black hole is extremely high, as all its mass is concentrated in a very small space. This makes black holes one of the densest objects in the universe. Compared to other celestial objects like stars or planets, black holes have much higher density due to their immense gravitational pull.
The densest thing in the universe is a neutron star, which is formed when a massive star collapses in a supernova explosion. Neutron stars are incredibly dense, with a mass greater than that of the Sun packed into a sphere only about 12 miles in diameter. Their density is so extreme that a teaspoon of neutron star material would weigh billions of tons on Earth. In comparison, neutron stars are much denser than other celestial objects like white dwarfs and even black holes.
Probably stellar mass black holes
The universe likely contains millions upon millions of black holes.