Glossary term: Antimatter

Description: In the early 20th century, theoretical physicists realized that for every kind of particle there should be a corresponding kind of antiparticle – a particle with the same mass, but otherwise completely opposite properties, in particular opposite electric charge. A few years later, the antiparticle of the electron was discovered: The "positron" has the same mass as an electron, but opposite electric charge. For some neutral particles, like the photon, the antiparticle is the same as the particle. When particle and corresponding antiparticle meet, they can annihilate to form photons. Our Universe appears to be made mostly of matter, not of antimatter consisting of antiparticles. The details of how that came about are the subject of ongoing research.

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