mystery

• "how it got out is a mystery"
• "it remains one of nature's secrets"

Synonyms

• enigmasomething that baffles understanding and cannot be explained; a difficult problem
• secretsomething that should remain hidden from others (especially information that is not to be passed on); information known only to a special group; something that baffles understanding and cannot be explained; not open or public; conducted with or marked by hidden aims or methods; not openly made known; communicated covertly; not expressed; designed to elude detection; hidden from general view or use; (of information) given in confidence or in secret; indulging only covertly; having an import not apparent to the senses nor obvious to the intelligence; the next to highest level of official classification for documents
• closed booksomething that baffles understanding and cannot be explained

Synonyms

• mystery storya story about a crime (usually murder) presented as a novel or play or movie
• whodunita story about a crime (usually murder) presented as a novel or play or movie