Question: In simple terms, what is antimatter?

Arttu Rajantie answered on 13 Jun 2011:

For most elementary particles there is a corresponding antiparticle, and antimatter is made of these. Not all, though, since there is no antiphoton, for instance. An antiparticle is identical to the corresponding particle in most ways, but it has the opposite electric charge. When a particle meets its antiparticle, they can destroy each other and produce photons and possibly other lighter particles.

Antiparticles really just come out of equations of quantum field theory, but there are different ways in which one can understand them intuitively. Paul Dirac, who first predicted their existence, explained them as “hole” in the vacuum from which a particle was removed. Richard Feynman interpreted them as particles going backwards in time.

It is obviously very difficult to handle antimatter because it cannot be kept in a container made of normal matter. Instead it is kept trapped in magnetic fields. There was some excitement very recently when physicists at CERN were able to produce hundreds of anti-hydrogen atoms and keep them trapped for 15 minutes.