Antimatter :

Antimatter is a substance composed of atoms made up of elementary particles that have the mass and charge of electrons, protons, or neutrons, their counterparts in ordinary matter, but for which the charge is opposite in sign. Such particles are called positrons, antiprotons, and antineutrons, or, collectively, antiparticles. Matter and antimatter cannot coexist at close range for more than a small fraction of a second because they annihilate each other with release of large quantities of energy.

Excerpt from the Encyclopedia Britannica without permission.