system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When NASA "spots" black holes, what do they actually see? Can it actually be seen or shown in an illustration?

EDIT: Thanks for all the answers! Almost every one of them explains or complements the other explanations. So you cannot SEE a black hole but you can observe it in relation to its environment i.e. how it affects everything around it.
gpt
They "see" them by what's happening around them. Matter that's falling into a black hole or orbiting very close to it heats up and emits radiation that we can see. Or, as with the supermassive black hole in the center of our galaxy, we can see stars which appear to be orbiting extremely quickly around something we cannot not see, but with an enormous amount of mass, which we infer by the motion and momentum of said orbiting stars.