Antimatter and matter annihilate each other upon contact, releasing the energy from both atoms. So the ex-antimatter would be half the released energy from the collision. I think?

Your equation is wrong. Why are you negating matter twice?

If you have anti-matter it will annihilate with matter making gamma rays. Those gamma rays can then participate in pair production making an electron and an anti-electron (positron), but that only happens in a strong magnetic field.

That positron will then annihilate again if it meets an electron (maybe even the same one, but it doesn't have to be). Which would then make gamma ray, which can start the process again. You can theoretically do this over and over forever, but realistically the gamma ray will probably be absorbed in something. Also, I think the annihilation may make two gamma rays, dividing the energy.

Edit: Is your equation a joke on the english words, ex and anti?

Actually, the annihilation mostly produces photons, which aren't either matter or antimatter (or both, whichever way you like.)

Though you might have been lucky and gotten an antineutrino instead. Now you just have to find it...