Antimatter

Antimatter is a kind of mirror image of ordinary matter. Every particle that atoms are made of has a counterpart with the same mass and exactly the opposite charge. The electron has the positron, the proton has the antiproton.

How does it work?

Ordinary matter and antimatter cannot exist together. When a particle and its counterpart meet, they both disappear and their energy is released as a flash of radiation. Antimatter is produced in tiny amounts in collisions in particle accelerators and in some forms of radioactive decay. It only lasts a short time there, because it almost immediately touches ordinary matter.

Why it matters

According to physics, matter and antimatter were created in almost equal amounts at the Big Bang. Had they been exactly equal, nothing would have been left over to make stars, planets and people from. So a small surplus of matter must have arisen. How that happened is one of the big open questions of physics.

A popular explanation revolves around neutrinos and their supposed heavy partners. Physicists at the University of Southampton and the Indian Institute of Technology Kanpur calculated that a heavy particle that has received hardly any attention may have made the young universe expand differently (link). That changes the sum behind the surplus of matter. New gravitational-wave detectors could partly test that idea. In this way, one of the oldest questions in physics is becoming step by step easier to investigate: why there is something rather than nothing.