What is antimatter?

For every particle of ordinary matter there is a corresponding antiparticle with the same mass but opposite electric charge — the antiparticle of the electron is the positron, for example. When a particle meets its antiparticle, the two annihilate, converting their mass entirely into energy according to E = mc². Antimatter is produced naturally in tiny amounts by cosmic rays and radioactive decay, and artificially in particle accelerators. One of the great mysteries of physics is why the universe is made almost entirely of matter, when the Big Bang should have created equal amounts of both. Even a gram of antimatter would be extraordinarily expensive to make.