Has anyone come up with a reason why gravity exists, or how it works?
My crack-pot theory is that all objects displace space, and it is this displaced space that causes objects to remain in orbit around one another.
I'm afraid that is, indeed, crackpot.
General relativity is the best current explanation of gravity. Object orbit one another moving in something that might loosly be called a "straight line" in spacetime but spacetime itself is bent by the mass of objects. Gravity is what we see from this bending.
The math of special relativity is pretty simple but GR is much, much harder (too hard for me). GR is also extremely well supported by many experiments. It is far and away the best answer we have right now.