poor benighted me, I was under the sad misapprehension that mysterium was a hypothetical substance to which a galactic radio emission at 1665(!) megahertz was attributed (until it was identified as an exceptionally strong component of a set of four lines emitted by the hydroxyl (OH) radical). this is, of course, not to be confused with mysterium tremendum, which is a term used to express the overwhelming awe and sense of unknowable mystery felt by those to whom this aspect of being is revealed.