Those are both great questions. I'll start with the easier one, the second.
Nuclear reactions don't create energy. They just convert energy from one form to another. The ones that seem to release energy start off with a lot of energy in a quiet form, as rest mass, and convert some of it to more violent forms, including light rays and fast motions of particles.
We don't fully know the answer to your other question. Some processes involving what's called the "weak nuclear force" happen differently to matter and antimatter. When the universe is changing rapidly those processes can leave more matter than antimatter. (Sakharov was the first to notice this possibility.) Although we know that the universe changed very rapidly after the Big Bang, the details of the earliest stages aren't yet known well enough to understand how the full matter-antimatter imbalance got going.
Mike W.
(published on 04/11/2012)
