What Drives Accelerating Expansion?

One of the phenomenon that has always confused me is the discovery that the Universe's expansion is accelerating. According to regular gravitational theory, this would be impossible, because the outside force acting upon all energy and light in this Universe should be pulling back toward the "big bang" point of origin. If its expansion is accelerating, that means another force is acting upon it.I read somewhere recently that some physicists thing it might be the 'anti-graviton'. But my understanding of anti-gravitons is they have the same properties (theoretically) of the graviton, meaning they attract just like regular gravitons.So do anti-gravitons repulse? If not, what other theories are out there as to why the universe is accelerating in its expansion?
- Jason Spielfogel (age 49)
Aliso Viejo

So far as I know gravitons and anti-gravitons are the same thing. 

We're used to the effects of gravity when we have two masses that are fixed. The gravitational energy is lowered when they move nearer each other. If you have a (finite) universe with many fixed masses, its gravitational energy is lowered when it contracts.

What if instead of having fixed masses you had some sort of fixed mass density so that as the (finite) universe expanded it had more total mass? It turns out then that expansion would lower the gravitational energy, so instad of pulling the universe together gravity would push it apart. 

General relativity (the modern theory of gravity) describes what happens to a universe filled with a fixed background  mass density regardless of whether the universe is finite or infinite. That background density still drives accelerating expansion. We call that apparent background "dark energy". 

Mike W.

(published on 05/01/2019)