Nine of these atoms were produced in collisions between antiprotons and xenon atoms over a period of three weeks. Each one remained in existence for about forty billionths of a second, travelled at nearly the speed of light over a path of ten metres and then annihilated with ordinary matter. The annihilation produced the signal which showed that the anti-atoms had been created.
Ordinary atoms consist of a number of electrons in orbit around an atomic nucleus.
They can only be created in the laboratory. Very occasionally, an antiproton converted a small part of its own energy into an electron and an anti-electron, usually called a positron, while passing through a xenon atom. Three quarters of our universe is hydrogen and much of what we have learned about it has been found by studying ordinary hydrogen. If the behaviour of anti-hydrogen differed even in the tiniest detail from that of ordinary hydrogen, physicists would have to rethink or abandon many of the established ideas on the symmetry between matter and antimatter. Comparisons can be made with tremendous accuracy, as high as one part in a million trillion, and even an asymmetry on this tiny scale would have enormous consequences for our understanding of the universe.