The existence of antiparticles was predicated by the gifted British physicist Paul Dirac in 1931. The first antiparticle to be discovered was the positron, whose existence was demonstrated in 1933. Learning of the positron discovery, Paul Dirac went on to predict that there must also be an antiproton. It was not until 1955 that the antiproton was produced in a particle accelerator. It is possible to picture a world consisting of only antimatter. In this world there would be anti-hydrogen, with the positron orbiting the antiproton of the atom’s nucleus. As far as astronomers can tell, our universe appears to consist entirely of matter rather than antimatter. Matter and antimatter annihilate each other when they meet and, according to E=mc^2, release vast amount of energy, equivalent to thousands of nuclear bombs’ output. It is believed that both matter and antimatter were created in almost equal amount soon after the Big Bang, but matter enjoyed a very slight edge. There were billion and one matter particles for every billion antimatter particles. For every billion particle-antiparticle pairs mutually annihilated, a single particle of matter survived. So, matter ended up dominating the universe.
Since it is conceivable the whole mirror-world of antimatter could exist, it would be vitally important to know whether a visitor from outer space came from a world of matter of antimatter before shaking his hand!