Allen Mills of the University of California, Riverside, and his colleagues say they have seen telltale signs of positronium molecules, made from two positronium atoms1.

Positronium is an other-worldly mimic of hydrogen. In a hydrogen atom, a negatively charged electron moves around a proton, which has a positive charge. The electrical force of attraction between the two subatomic particles holds them together.

In positronium, hydrogen’s proton is replaced by a positron, which is the antimatter partner of an electron. A positron has the same positive charge as a proton, but has the same mass as an electron, which is just 1/1,836 that of a proton. So positronium is an extremely light ‘atom’.