Physic student here, so not a full professional yet.

I just wanted to add to this nice explanation that using the models of how we expect gravity to behave, we theorized the existence of black holes way before observing them. And not just black holes, but many other observations about the nature of our universe were theorized before we saw them. Like light bending.

That’s how good the math was, that the models keep proving to be accurate as we observe more of our universe. I find that so cool :)