We are talking about what existed "before" (quoted because we have no correct word to accurately describe the relationship) spacetime itself. A brane is not an object made of matter like everything around you. It is an extradimensional construct with characteristics totally unlike what you see in classical physics.
So it could in fact be a wind-up universe generator, or an extra-dimensional goblin or two jube-jubes rubbed just the right way. It could be anything other than nothing. So the problem still remains, something came from nothing...