Matter composed of the counter-parts of ordinary
matter (they're their
charge conjugates, i.e.,
they have the
opposite electrical charge and
magnetic moment):
Antiprotons instead of
protons.
Positrons instead of
electrons.
Antineutrons instead of
neutrons.
When matter and antimatter collide, both may be annihilated, and other
elementary particles, such
as
photons and
pions, are produced.
In
1932 Carl D. Anderson, while studying
cosmic rays, discovered the
positron, or
antielectron, the first known
antiparticle.