mysterium

noun2

1. •
   (historical) Any of various unknown elements thought to make up existing forms of matter, or a substance seen as an elemental or pure form of something else.

   • There are many such mysteria: milk is a mysterium of cheese and butter, and cheese in turn a mysterium of maggots, which were thought to form spontaneously in rotting food. — 2006
2. •
   (historical) The hypothetical source of a galactic radio emission at 1665 megahertz (later identified as due to hydroxyl radicals in interstellar space).