Black holes are a pretty controversial topic. Generally anything that we look for in the universe may not be certain. Black holes do not emit and kind of energy or light from the spectrum, so how can we even know that they may exist. It is based on a scientific hypothesis that is not known for sure yet. Also, Einstein made theories about the gravity from black holes. He said that the gravity is so strong that time itself is bent. If one flew near a black hole, time would seem to be moving in double the time it would normally. This would make time travel possible, however one would only be able to travel into the future. This is why black holes are so controversial.