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.
The principles on which these divisions were made remain an enigma to the present day.
Man is a hopeless enigma to himself, until he sees himself in the light of being a human with human failings.
He did a lot of research so he is not confused by the enigma of the pain that comes with the disease.