Alasdair Wilkins in io9:
Positronium is a particle created when you bind together an electron and its antimatter counterpart, the positron. It doesn't interact with other atoms in the way we would expect, and this discovery could help us solve the universe's biggest mysteries.
Positronium is sort of like a hydrogen atom, except if you took away the lone proton in the nucleus and replace it with a positron. Because electrons – and, by extension, positrons – are only 1/1836 the mass of a proton, that means positronium particles are much less massive than their hydrogen counterparts. The particle is a common byproduct of the interaction between regular matter and positrons. It's an unstable particle, only remaining together for an average of 142 nanoseconds before decaying into two gamma ray particles.
During their very short lifetimes, however, it's possible to probe some of their properties and characteristics, and that's what researchers at University College London recently attempted. They tried a scattering experiment, in which they sent streams of positronium particles at different atoms and molecules and measured how they interacted. Because positronium is a hybrid of an electron and positron, they expected the particle to act in a way that was some sort of average of these two.
But that isn't what they found. Instead, positronium acted precisely the same as an electron would. That doesn't make much sense – electrons are negatively charged, while positronium is neutral, and it's obviously twice the mass of a lone electron. In some weird way, the effects of the positron's presence seems to be cloaked so that only the electron half of the positronium interacts with other matter.