I think the simplist answer is we know NOTHING about gravity. there is only theory on what gravity actually is... all we know are its effects. for example the mayans knew with amlost perfect accuracy about the suns behavior to such a degree that they could predict the earths presession. Buut they had absolutly no knowledge of anything ABOUT the sun, like nuclear fusion and photons. That is pretty much where we are at the moment. We understand gravity's behavior with an extraordinary degree of accuracy, however we know absolutly nothing about gravity.