A large accumulation of antiparticles - antiprotons, antineutronsand positrons (antielectrons) - which have opposite properties to normal particles (e.g. electrical charge), and which can come together to make antiatoms. When matter and antimatter meet, they self-destruct in a burst of high-energy photons or gamma rays. The laws of physics seem to predict a pretty much 50/50 mix of matter and antimatter, despite the observable universeapparently consisting almost entirely of matter, known as the “baryon asymmetry problem”.