Is the theory of gravity just a "theory"?

3D Representation of mass curving spacetime

Did you know that there is no such thing as a gravitational force?  Nothing is "at rest" And nothing "attracts"  anything else as if it were a magnet,  everything simply moves through on a straight line on a geodesic.  Before this can be grasped you have to check out this super-cool video on frames of reference below,  made in 1960. The sound takes a few seconds to kick in.

It can be expanded upon in thought experiment.
If if there was only a single particle in a universe with no fields,  would it be still or moving?  The answer is that it's not knowable or relevant. The one particle universe has no space.  Movement t is not possible and time cannot pass because causality requires a sequence of particle events. 
Add another particle.  The two particle universe only allows the particles to move closer to each other or further apart.  Even if you imagine the space around them in your mind to be 3 Dimensional it is irrelevant,  that's just the way we think,  only the space between the particles matters,  other dimensions don't exist there.  Time can at least now pass.
In a three particle universe all the complexities of our universe emerge as per the above video.

But we still need to add fields! And we still need to add a fabric framework for spacetime. What is it?  It boils down to the path light takes travelling through spacetime,  but there is a massive difference (so to speak)  between trapping a moon into orbit and trapping a bean of light.

Hopefully you will be interested enough to watch the 5 series of clips below to resolve the finer points of this topic.