I'm a little confused over what constitutes negative and positive energy. I've heard it said that we have a zero energy universe because the mass of the galaxies, etc. counts as positive energy which is exactly balanced out by gravity, or the gravitational potential, which counts as negative energy. Ok, that's understandable. But what about negative energy as in "antimatter." The negative energy solutions to say, the Dirac equation, talk about negative energy electrons with negative mass. This type of negative energy seems qualitatively different from a negative energy which seems equivalent to "potential energy." What's going on here?