Absolutely anything and everything. They may even suck up time(!!!!!), but no one knows for sure.
Black holes. They can be so large that they can suck up universes at a time
yes
Yes, all black holes 'suck stuff up'.
Yes
yes but not likely
Yes.
White holes are theoretical regions of spacetime that expel matter and energy outward, the opposite of black holes which pull matter in. They do not suck up objects like black holes do. However, there is no observational evidence for the existence of white holes in the universe.
Black holes were stars that were so massive that they collapsed on itself. The gravity in black holes is infinite and more you get closer to it, more time gets slower. Black holes suck all matter that is too close. Even light can't escape Black holes.
You can't because they suck in light, there for you cannot see them
There are hypotheses about so called 'virtual particles' that may travel faster than speed of light, and hence are not sucked up by Black Holes. Also, Black Holes cannot suck another bigger Black Hole, when they meet a bigger one, they get sucked up rather.
Black holes do not emit light, so black holes can not be seen this way. But black holes emit X-rays, but stars are not hot enough to emit X-rays. When black holes suck up stars, energy goes to the black hole, and come out as X-rays.
Black holes have such strong gravitational pull that they can theoretically pull in anything that gets too close, including people and rocket ships. However, the likelihood of a person or a rocket ship getting close enough to be pulled into a black hole is extremely low, as we are very far away from any known black holes.