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What is antimatter?

Slow down! "Antimatter?" "Pure energy?" What is this, Star Trek?

The idea of antimatter is strange, made all the stranger because the universe appears to be composed entirely of matter. Antimatter seems to go against everything you know about the universe.

Image courtesy of Fermilab

But you can see evidence for antimatter in this early bubble chamber photo. The magnetic field in this chamber makes negative particles curl left and positive particles curl right. Many electron-positron pairs appear as if from nowhere, but are in fact from photons, which don't leave a trail. Positrons (anti-electrons) behave just like the electrons but curl in the opposite way because they have the opposite charge. (One such electron-positron pair is highlighted.)

If antimatter and matter are exactly equal but opposite, then why is there so much more matter in the universe than antimatter?

Well... we don't know. It is a question that keeps physicists up at night.

(The usual symbol for an antiparticle is a bar over the corresponding particle symbol. For example, the "up quark" u has an "up antiquark" designated by , pronounced u-bar. The antiparticle of a quark is an antiquark, the antiparticle of a proton is an antiproton, and so on. The antielectron is called a positron and is designated e+.)