Every particle in existence has an antiparticle equivalent, which is almost identical except it carries the opposite electric charge. Matter is composed of normal particles and antimatter is composed of antiparticles—for example, while a proton and an electron form an ordinary hydrogen atom, an antiproton and a positron form an antihydrogen atom. Antimatter is created all the time in high-energy collisions, like when cosmic rays impact Earth’s atmosphere, but it immediately disappears because when matter and antimatter collide, they annihilate in a flash of pure energy. This makes it difficult to study experimentally, and neither can we find any evidence of a significant concentration of antimatter in the wider universe. The universe we know is dominated by ordinary matter—it makes up every person and planet and star—and yet if matter and antimatter were created equally at the birth of the universe, where has all the antimatter gone? This asymmetry is a perplexing question in physics, and several theories have been proposed to explain it. Perhaps nature favours matter reactions over antimatter ones; or perhaps matter and antimatter particles decay differently; or perhaps there are far flung regions composed primarily of antimatter, but they’re just beyond our visible universe. Researchers are currently trying to determine if such regions exist by studying colliding superclusters for high-energy signatures of annihilation, and by studying decay patterns in quarks.