Re: the nature of gravity. Newton said frankly that he didn't know what its mechanism is. He tended to think that it might be some form of magnetism, but he left that up in the air (so to speak). Einstein theorized that it is the warping of space around a object, allowing objects to "roll in" toward the center (which of the two objects moves how much depends on the relative mass of the objects). But that is not locked in stone (sorry about the underline, but if I were saying this rather than writing it, I would vocally emphasize that word). "Gravitons" (like photons) have been hypothesized, but no one has yet found one, so we don't even know if they exist. Same sort of thing as neutrinos: a mathematical construct to balance a formula--until they figured out a way to look for them and, lo and behold, they found them.

The goal posts (e.g., the answer to the question "What, exactly, is gravity?") haven't moved an inch. We're still trying to locate where they are.

Don Firth