mysterium
/mɪˈstɪərɪəm/Astronomy. Now historical
noun
A mysterious element; specifically a hypothetical substance to which a galactic radio emission at 1665 megahertz was formerly attributed, later identified as an exceptionally strong component of a set of four spectral lines emitted by hydroxyl radicals in interstellar space.
Origin
1960s. Either from myster- + -ium, or from classical Latin mystērium mystery.