That's actually a very good question. I suppose I could have started the conversation anywhere. I guess in the back of my mind I suspected that when it came to identifying the actual "cause" of the expansion of "space" (whatever that is), it was going to sound something like:

It is profoundly impossible for the ad hoc "invisible cheeseburger force" to not exist, if we assume the validity of the expansion of space for the simple reason that "invisible cheeseburger forces" are, by definition, the cause of that effect.

I've not had time to really give Tim's first reference a thorough read through, but already I see things headed in that direction. I'll comment more when I've been through the paper, and I have a better idea of how the expansion of "space" (whatever that is) presumably affects real life "spacetime" according to these papers. Are the spaces inside of atoms "expanding" too, or is this a special "peek-a-boo" force that is shy around science labs like DE?