En física de partículas, la antimateria es la extensión del concepto de antipartícula a la materia. Así, la antimateria es una forma de materia menos frecuente que está constituida por antipartículas en contraposición a la materia común que está compuesta de partículas. Por ejemplo, un antielectrón (un electrón con carga positiva, también llamado positrón) y un antiprotón (un protón con carga negativa) podrían formar un átomo de antimateria, de la misma manera que un electrón y un protón forman un átomo de hidrógeno. El contacto entre materia y antimateria ocasiona su aniquilación mutua, esto no significa su destrucción, sino una transformación que da lugar a fotones de alta energía, que producen rayos gamma, y otros pares partícula-antipartícula.

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In particle physics, antimatter is a material composed of antiparticles, which have the same mass as particles of ordinary matter but opposite charges, as well as other particle properties such as lepton and baryon numbers. Collisions between particles and antiparticles lead to the annihilation of both, giving rise to variable proportions of intense photons (gamma rays), neutrinos, and less massive particle–antiparticle pairs. The total consequence of annihilation is a release of energy available for work, proportional to the total matter and antimatter mass, in accord with the mass–energy equivalence equation, E = mc2. Antiparticles bind with each other to form antimatter, just as ordinary particles bind to form normal matter. For example, a positron (the antiparticle of the electron) and an antiproton (the antiparticle of the proton) can form an antihydrogen atom. Physical principles indicate that complex antimatter atomic nuclei are possible, as well as anti-atoms corresponding to the known chemical elements. Studies of cosmic rays have identified both positrons and antiprotons, presumably produced by collisions between particles of ordinary matter. Satellite-based searches of cosmic rays for antideuteron and antihelium particles have yielded nothing. There is considerable speculation as to why the observable universe is composed almost entirely of ordinary matter, as opposed to an even mixture of matter and antimatter. This asymmetry of matter and antimatter in the visible universe is one of the great unsolved problems in physics. The process by which this inequality between particles and antiparticles developed is called baryogenesis. Antimatter in the form of anti-atoms is one of the most difficult materials to produce. Antimatter in the form of individual anti-particles, however, is commonly produced by particle accelerators and in some types of radioactive decay. The nuclei of antihelium (both helium-3 and helium-4) have been artificially produced with difficulty. These are the most complex anti-nuclei so far observed.

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