mys·ter·y

noun
1. anything that is kept secret or remains unexplained or unknown: the mysteries of nature.
2. any affair, thing, or person that presents features or qualities so obscure as to arouse curiosity or speculation: The masked guest is an absolute mystery to everyone.
3. a novel, short story, play, or film whose plot involves a crime or other event that remains puzzlingly unsettled until the very end: a mystery by Agatha Christie.
4. obscure, puzzling, or mysterious quality or character: the mystery of Mona Lisa's smile.
5. any truth that is unknowable except by divine revelation.

usage

The inner workings of a woman's brain are a complete mystery to me.

mys.ter.y