Why are black holes so difficult to study?
By definition, they cannot produce light, so they're elusive from that point of view. They are also the most compact objects that can be produced – if you could compress the Sun to a radius of about three kilometres, then you would produce a black hole. Because they are intrinsically compact and normally at very large distances, their projected size on the sky is extremelywhich is why it's so hard to see them.