From The Economist:
Readers who were paying attention in their maths classes may recall that quadratic equations often have two solutions, one positive and one negative. So when, in 1928, a British physicist called Paul Dirac solved such an equation relating to the electron, the fact that one answer described the opposite of that particle might have been brushed aside as a curiosity. But it wasn’t. Instead, Dirac interpreted it as antimatter—and, four years later, it turned up in a real experiment.
Since then antimatter—first, anti-electrons, known as positrons, and then antiversions of all other particles of matter—has become a staple of both real science and the fictional sort. What has not been available for study until recently, however, is entire anti-atoms. A handful have been made in various laboratories, and even held on to for a few seconds. But none has hung around long enough to be examined in detail because, famously, antimatter and matter annihilate each other on contact. But that has now changed, with the preservation of several hundred such atoms for several minutes by Jeffrey Hangst and his colleagues at CERN, the main European particle-physics laboratory near Geneva.