The article only mentions Muonium in passing -- I guess it's only somewhat exotic? -- but I can assure you that it's very real. Chemically it acts as a light is

The article only mentions Muonium in passing -- I guess it's only somewhat exotic? -- but I can assure you that it's very real. Chemically it acts as a light isotope of hydrogen, and the interaction of the (anti)muon spin makes it useful for probing reactions in otherwise problematic environments -- for example, supercritical water, where the pressure chamber needed for hundreds of atmospheres of pressure gets in the way of most sensors.