Lemme jump in, while we're waiting for the smart guys to compose their answers. The idea of these branes is that gravitons are traveling here from the other brane, where gravity is its "normal" (much stronger) self. I get the idea that when this happens, stuff from here must go to the other brane, so as to balance the cosmic books. These arrivals and departures should be, in principle, detectable. But the other brane is thought to be so close to ours (maybe one planck length, says Lisa) that the transit time is probably negligible. So if all we can detect is that everything balances, and there's no detectable moment when we've got a shortage on its way to being compensated, then we've got a null result. But I may have this all messed up. Let's wait for the experts.
This is familiar to me. Back when I was an accountant, I had a "plug" account where things that didn't balance would disappear. OH MY GOD! The Creator is a CPA...
parsy, the former zen accountant.