Because black holes are very hard to see, people trying to see them look for them by the way they affect other things near them. The place where there is a black hole can be found by tracking the movement of stars that orbit somewhere in space. Or people can find it when gas falls into a black hole, because the gas heats up and is very bright. This can be found by Earth or Earth-orbiting telescopes. Inside a black hole the rules of physics are very different.