In my opinion, though not as an informed expert on the topic - no. Why? Two reasons 1) I'm sure someone would have thought of it already 2) The gravitons passing brane to brane would be a weak 'short-lived' "source" of gravity on our plane, e.g. we'd be able to see their gravitational pull coming in and out on the brane. Imagine a light in a room, if the light is passing in and out of the room on some form of oscillator the room will sometimes be light and sometimes be dark, right? I will, however, have a big ponder on it and see if i can come up for any more arguments pro and against. Oh, big point though, we know that dv/dt (Universe) is positive, which means if we allow for the existance of the concept of dark energy, we know there is an inherent suction to the universe ergo the 'amount' of dark energy in the universe is proportional to a function of the space occupied. If you imagine we have a property of the space and we are increasing in space, it'll only increase. However, with allowance for a graviton exchange system, it's, to me, looking more like an unsteady wave, and not to mention we wouldn't be able to account for the increase in v?

I totally think it's possible. Actually, I think I read this somewhere else...possibly The Elegant Universe or The Fabric of the Cosmos? I'm pretty sure Brian Greene mentions it in passing in one of those. There are a lot of places it could come from. Who's to say that it's not from something that exists in the 6 or so dimensions that we can't see, experience, or study? Or that dark matter isn't just matter existing in those dimensions? Or even that gravitons exist? They're completely theoretical. That's why gravity is so complicated. When we have a consistent theory of gravity, we'll be only one step away from a ToE.

Don't read this, I scrolled down too far and typed this into the 'answer' box' and not the 'comment box'! Surely, like a vacuum, the overall effect of gravitons passing freely from brane to brane will seem stable on the macro-level? dv/dt is positive at the moment but it is theorized that it isn't constant and can change over time to be negative.