mysterium

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.

The hypothetical source of a galactic radio emission at 1665 megahertz (later identified as due to hydroxyl radicals in interstellar space).