Time-reversal invariance and the CP violation are connected to another asymmetry of the universe, the imbalance between matter and antimatter. At the microscopic level, matter and antimatter are always created together in 1:1 correspondence. High energy collisions produce equal numbers of quarks and antiquarks. And yet, our universe has a conspicuous surplus of matter, of which we and our surroundings are made. How did this happen?

A clue to this deep mystery is provided by the CP violation in the K^o meson, which shows decay modes having a preference for matter over antimatter. The K^o does not have enough mass for its decay to produce protons, but its decay asymmetry suggests that some more massive particle, perhaps a B^o meson containing a bottom quark, might in the early universe have decayed preferentially into protons rather than antiprotons, leading to the present day dominance of matter. Future experiments using the B-Factory, presently under construction at the Stanford Linear Accelerator Center (SLAC), will investigate this problem.

Antimatter exists in nature only in the form of antiprotons present in very small numbers in cosmic rays and in positrons (antimatter electrons) produced in some radioactive decays. Recently, evidence has also been found for a "fountain" of positrons ejected from some object near the center of our galaxy, presumably a black hole.

However, we are getting better and better at producing and storing antimatter in the laboratory. Antiprotons, antineutrons, and even antideuterons (a nucleus consisting of an antineutron and an antiproton) are routinely produced using high energy particle accelerators at Fermilab in Illinois and CERN in Geneva, Switzerland. Positrons and antiprotons have been trapped in electric and magnetic fields and held under high vacuum for several months. Recently, "antihydrogen" atoms having a positron orbiting an antiproton have been formed in laboratory experiments. These researchers are looking for any indication that trapped positrons, antiprotons, and antihydrogen atoms show a behavior that differs in any way from that of their normal matter counterparts, because any such difference would represent a violation of CPT symmetry.

Antimatter nuclei are also interesting for other reasons. Special facilities at CERN and Fermilab provide beams of low energy antiprotons and permit nuclear scientists to study the interactions of antiprotons with matter. While a positron and an electron usually annihilate to form a pair of gamma-ray photons traveling in opposite directions, the annihilation of an antiproton with a proton is more complicated. Several p mesons are usually produced. About a third of the mass energy of the proton-antiproton pair becomes inaccessible in the form of energetic neutrinos.

Nevertheless, antimatter can be viewed as an extremely compact form of stored energy that can be released at will by annihilation with matter. The US Air Force has commissioned design studies of antimatter-powered space vehicles that, given a supply of antimatter, look quite feasible. The problem with such schemes is that production of any significant quantity of antimatter would cost far too much right now to be economically feasible.