What I would like to find... would be theories trying to guess the properties/source of this force... and what influences it should effect -- on itself, conventional matter and conventional energy.
I would hope to find an informed guess... of how it might behave (especially differences of behaviour with regard to gravity) and thereby try to imagine what it might be and do.
Okely-dokely. Here goes.
Start with M-Theory (what used to be called String Theory). You can find out more about it from
The Elegant Universe
Okay, now you understand M-Theory, right?
Well, here's my theory regarding the expansion. Remember, I am
not a physicist, nor do I claim to understand any of it. But you asked for a theory, and this isn't a bad one, for science fiction purposes.
If you read up on M-Theory, you'll notice that the 'standard' model posits a series of 'branes' or universes, each more or less parallel to each other, like slices of bread in a loaf. Occasionally, the oscillations of these branes causes one to bump into another. Some M-Theory specialists posit that such a collision might have been the genesis of the Big Bang.
Another feature of this model is that it might possibly serve to explain why gravity is so vastly weaker than every other fundamental force in the universe. It could simply be that while those other forces are confined to a single brane, gravity radiates out across the set of branes and thus is only weakly felt in any one brane. Dunno if that's true, but it feeds into my theory, so I'll grab it!
So, picture this. Something, maybe a brane collision, creates our universe some 13.5 billion years ago. Then something else speeds up its expansion some 6 billion years ago. Could this second something be a near-collision? Perhaps the gravitational field of the brane we nearly collided with 6 billion years ago was strongly felt on our brane, causing the visible universe to rapidly expand? Hell, I dare you to prove differently! And if you do, I'm sure I won't understand it, anyway!
Send those dandelion pictures to Hypatia, please.