Antimatter is a type of matter composed of antiparticles, which have the same mass as particles of ordinary matter but opposite electric charge and other quantum properties. For example, the antiparticle of the electron is the positron, which carries a positive charge instead of a negative one. Similarly, the antiproton is the antiparticle of the proton and has a negative charge.
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Antimatter is an assembly of antiparticles. A cosmological antimatter population is distinct from the small secondary antiparticle populations produced by energetic collisions.
Predicted by the Dirac equation.
Can be easily seen from the solution of Equation "Expanded Dirac equation in Planck units" when the particle is at rest as shown at Video "Quantum Mechanics 12b - Dirac Equation II by ViaScience (2015)".