People keep talking about gravity as a force; one of the four forces in nature. They try to combine it with the other three forces and cannot... and wonder why it is so weak compared to the electromagnetic force or the strong nuclear force.
The familiar drop of an apple from a tree (the apparent 'force' of gravity) is merely the behavior of two massive objects which are able to move in relation to each other within this structure of space-time. Our human experience observes motion and gravity with the perspective of distance and duration, accelleration and inertia, so we naturally label it a force.
I'm thinking that gravity is not a 'force' at all; not it the cosmological sense. It does have the attributes or behavior of a force when two or more objects having mass move close to each other. But doesn't it make more sense to think of gravity as a dimension of space-time? It is a predictable flexation of the fabric of space-time given the presence of mass.
The dimension of gravity regulates and gives structure to the space-time continuum. It balances matter and energy states. Without space-time, matter-energy has no place to go and nothing to do. Without matter-energy, space-time couldn't exist. The dimension of gravity explains what the universe is and what it does on a large scale. It isn't any 'thing' or 'force' per se, and yet it is the prime ingredient of everything.
But what is it? When trying to explain the propagation of light waves through space, scientists invented a substance called the 'luminiferous ether'. Which turned out didn't exist and wasn't needed. Now scientists have come up with 'dark matter' and 'dark energy' to try and explain the increase in the expansion of the universe. Just a few years ago, no one could guess whether we were living in a closed universe or an open one. Now they are trying to detect sub-atomic particles to explain this mysterious energy and/or matter that can't be seen, only theoretically surmised. They keep waiting for the discovery of the graviton which would mathematically connect the world of the very large with the world of the very small through the theory of quantum gravity.