system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The proof behind the existence of black holes.

The proof used to know about their existence before they were captured on camera.
gpt
To understand it, first you have to understand escape velocity.

If you throw a ball straight up, depending on the velocity the ball it has some velocity. It will be dragged on by the gravity of the Earth to slow it down and eventually it will stop and then fall back. However, the farther the ball gets from Earth, the weaker the gravity. If you throw the ball fast enough (faster than you can throw) gravity will get weaker faster than it can slow down the ball and in that case, the ball will never stop and fall back, it will just get slower and slower and slower forever. The minimum speed to achieve this from some distance from a gravity well is the escape velocity. The stronger the gravity, the faster the escape velocity.

We already had evidence for super-dense stars which we would later call neutron stars. The question just became, what would happen if a star was so heavy and so dense that the escape velocity from its surface was greater than the speed of light? Even light would be unable to achieve escape velocity, so anything which passed this 'event horizon' would be lost from the rest of the universe forever.