The famous physicist Richard Feynman said (either once or many many times, I'm not sure) that you can think of an antiparticle as simply a 'particle moving backwards in time'. I thought that was a neat little hint at some of the inner workings of elementary particle physics. It explains why EVERY*** particle has an antiparticle, since if one particle can move backwards in time, presumably any particle can.

There are also some neat little tricks in the math that make this whole time-reversal thing seem more real. Hole theory says that antiparticles have 'negative energy' -- negative rest mass and negative kinetic energy. There's a term in time-dependent wave functions that looks like exp(-i*E*t/hbar). If you stick in a negative value for E, and a negative value for t, corresponding to an antiparticle wavefunction, you're back to the same thing! (-E)*(-t)=E*t ..kinda cool, ain't it?

***Assuming there's some way to distinguish between the two. With the photon, there isn't, since it's charge-neutral.. this is why no one ever talks about antiphotons. But in general, there do exist antibosons, mind you. The two W bosons (+ and -) are antiparticles of each other.

Oh, and another thing.. When a particle and its antiparticle annihilate (Let's say they're both at rest or have minimal kinetic energy, for simplicity), the energy of the resultant photon is 2*mc^2, which is the difference between +mc^2 and -mc^2.. and if you think of this transition as analogous to the process of an excited atom going to its ground state and emitting a photon in the process, this possibly implies that THE VACUUM itself is the entity that is transitioning.

Whoa, maybe I've gone plain silly, but then again, maybe not.