Explain the RE mechanism for gravity, then.
I can see what you're getting at...
We say that all objects that have mass attract each other with a force that is proportional to the two masses and inversely proportional to the distances between their centers squared, and then you say but then what is the mechanism behind the gravitational force, knowing full well that gravitons have not yet been discovered.
Well, by the same argument, I could say: What is the mechanism behind a force caused by a net overall acceleration? ie. Why is f=ma. I'm sure you will tell me something along the lines of.... all matter resists change, so the "Force" we experience is simply a resistance to a change in velocity.
Well why is that then? Because that is our experience of the universe? Well it is also my experience of the universe that all matter exerts an attractive force on all other matter.
My point is that in both of our theories, and almost any scientific theory, certain concepts need to simply be taken for granted before they can be expanded upon in order to give a meaningful picture of the mechanisms that drive our universe.