By Oleksandr Koliakin
The positronium is an exotic atom, made out of an electron and an antielectron (positron) orbiting around each other. Unlike in 'normal' atoms, where leptons orbit a nucleus made out of baryons or mesons, in positronium atoms the electron and positron are sort of 'chasing each other in circles'.
The chemical representation of positronium is Ps.
Atoms (if we can call them that) of positronium can appear when a positron that is slowed down while traveling through matter is captured by an electron. However, positroniums are highly unstable, the particles that it is made out of usually annihilate each other. In fact, the mean half life of a positronium is about one-tenth of a nanosecond (and a nanosecond is one billionth of a second)
Sources:
Encyclopedia Britannica (https://www.britannica.com/science/positronium)