I have thought for some time that perhaps the Big Bang actually created two universes, ours and an anti-universe made up of anti-matter which expanded away from ours at the instant of creation. Richard Feynman actually played around with the idea that anti-matter ran backward in time. Such a duel universe theory would thus explain why anti-matter is so rare in our universe rather than being 50% of ours. Virtually all of the apparently missing anti-matter is possibly in the anti-universe. Also, since the two universes were expanding in what mathematically could be thought of as opposite directions, the net energy required for the expansion would be zero since they would balance each other out.