Anything that arouses curiosity or perplexes because it is unexplained, inexplicable, or secret
Anything or anyone inexplicable
My past is such an enigma; I don't know any of the details.
It was the most perplexing enigma in astronomy.
Some clue to this enigma might be found by asking a lot of questions.
The reason that the source of the noise is such an enigma is that no one ever traced the sound when they heard it.
Man is a hopeless enigma to himself, until he sees himself in the light of being a human with human failings.