It is very hard to see a black hole. Any object that gets too close to a black hole will be pulled inside it. We only know they are there because of the effects they have on other objects that are near them. Any object, whether some dust, or a star, or anything, that gets too close to a black hole will be pulled inside it. As the objects fall toward the black hole, they heat up and get very hot.
Scientists can use special instruments to detect the heat the objects give off.
That is how we know the black hole must be there. Show me the Level 2 version of this page. Show me a printable version of this page.