Are black holes a theory of science or a fact?
The theory of black holes comes from Einstein's General Theory of relativity. They were originally theoretic, but more recent observations have discovered objects which can only be black holes.
The physicist Karl Schwartzschild produced the first exact solution to Einstein' equations of relativity. This solution described what we now call a black hole.
Schwartzchild's solution defines the Schwartzchild Radius which if a body is compressed to its Schwartzchild radius the escape velocity from the surface would be the speed of light. This means that even light can't escape.
The term black hole was coined to describe a body which has all of its mass confined within its Schwartzchild radius. The existence of black holes wasn't accepted for some time.
The term black hole was chosen because the body can't radiate light as light can't escape from its surface. In fact a black hole emits a lot of light at all frequencies. This is because material spiralling into the black hole gets superheated by gravitational and frictional forces which cause it emit light light. This material spiralling into a black hole is called an accretion disc.
More recent observations have observed that galaxies, including our own, have super massive objects at their centres. These objects can only be black holes because of the light emitted is consistent with them having an accretion disc. So, even though a black hole has never been observed directly, the scientific community now accept that they exist.