[I see that gravity remains a mystery. I wrote about it in Hollow Planets in 1999, and indeed, as I said, it is a huge mystery for science to this day. Here is a short excerpt explaining how little is known about it. Jan]
What the heck is gravity, anyway?
What is gravity, anyway? Other forces are mediated by particles. Electromagnetism, for example, is the exchange of photons. The weak nuclear force is carried by W and Z bosons, and gluons carry the strong nuclear force that holds atomic nuclei together. McNees said all of the other forces can be quantized, meaning they could be expressed as individual particles and have noncontinuous values.
Gravity doesn’t seem to be like that. Most physical theories say it should be carried by a hypothetical massless particle called a graviton. The problem is, nobody has found gravitons yet, and it’s not clear that any particle detector that could be built could see them, because if gravitons interact with matter, they do it very, very rarely — so seldom that they’d be invisible against the background noise. It isn’t even clear that gravitons are massless, though if they have a mass at all, it’s very, very small — smaller than that of neutrinos, which are among the lightest particles known. String theory posits that gravitons (and other particles) are closed loops of energy, but the mathematical work hasn’t yielded much insight so far.
Because gravitons haven’t been observed yet, gravity has resisted attempts to understand it in the way we understand other forces – as an exchange of particles. Some physicists, notably Theodor Kaluza and Oskar Klein, posited that gravity may be operating as a particle in extra dimensions beyond the three of space (length, width, and height) and one of time (duration)we are familiar with, but whether that is true is still unknown.