Lucia Halpern, age 16, of Charlotte, N.C., for her question:

WHAT IS ANTIMATTER?

Antimatter is matter composed of atomic particles that are the opposite of ordinary atomic particles. These opposite particles are called antiparticles.

Physicists have produced various antiparticles, including positrons, antineutrons and antiprotons. All these antiparticles are made in a cyclotron or some other high energy particle accelerator.

But scientists have not yet observed antiparticles combined into antiatoms. Many scientists believe antiatoms could exist, because some antiparticles do combine.

If a lump of antimatter composed of antiatoms existed, it would look exactly like a lump of ordinary matter. But if the two lumps came together, they would disappear and produce much energy. For this reason, antimatter could never be found on the earth.