Question

Why was it so hard to prove that black holes exist?

Answer 1

They are invisible.

Explanation:

Since black holes have such high gravity that light does not escape there is no way of seeing them. The only way they can be observed is by the effect they have on other things. For example, one effect that has been observed is called a gravitational lens which occurs when a black hole passes between the Earth and a bright object. In this case the high gravity bends the light as it passes near the black hole.

This is a simulation of a black hole passing in front of a galaxy.