there are many theories that run the gamut from relativity to Hawking's theory that black holes evaporate.

If you are writing in broad terms, I would stick with gravity first. Then a very little bit about stellar masses (in order to compare to black hole masses)

Also I would include information about searching for these holes. Since light cannot escape, how do we find them? -By tracking the velocities of stars orbiting around an unseen mass.

just some thoughts