If you can’t see a black hole since light itself gets sucked into it, then how do you see it?  Well, you guess where it is because stars might be orbiting it and you can still see the stars orbiting something that you can’t see.  You can guess that it’s a black hole.   That’s how we figured out there’s a super massive black hole there. Watch from 22:38 to 26:37.

https://www.youtube.com/watch?v=cKSncLxC9lc

You can also see black holes using telescopes that can use x-rays, like the Hubble Space Telescope.   This is an image of a black hole ——————->