un mystère
Consciousness is still a mystery, for both science and philosophy.
La conscience reste un mystère, tant pour la science que pour la philosophie.
Despite the importance of sleep, its purpose is a mystery.
Malgré l’importance du sommeil, sa fonction reste mystérieuse.
I think the reason that he killed his wife is still a mystery.
Je pense que la raison pour laquelle il a tué sa femme reste un mystère.
Why he killed himself is still a mystery.
Les raisons de son suicide restent un mystère.