The antimatter version of the hydrogen atom could soon finally give up its secrets. Scientists expect that antihydrogen will have exactly the same properties as hydrogen; but after 80 years, the test is only just becoming possible.

Every particle has an antiparticle, which is identical in every respect except that it has opposite charge. The negatively charged electron has the positron, and the proton has the antiproton. Together, an antiproton and a positron form the simplest anti-atom, antihydrogen. Once the anti-atom is formed, it must be kept apart from normal matter. When a particle and its antiparticle meet, they destroy each other, turning into energy in a process called annihilation. That gets to the heart of the biggest mystery about antimatter. When the Universe formed, equal amounts of matter and antimatter should have formed; but if that were the case, they should have annihilated each other since.