We don't have synonyms for mysterium yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are mysterious, mysterial and mysteried.
(n) :
(chemistry, alchemy, now 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.
(n) :
(astronomy, now historical) The hypothetical source of a galactic radio emission at 1665 megahertz (later identified as due to hydroxyl radicals in interstellar space).
(n) :
(chemistry, alchemy, now 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.
(n) :
(astronomy, now historical) The hypothetical source of a galactic radio emission at 1665 megahertz (later identified as due to hydroxyl radicals in interstellar space).