enigma
n.something or someone that is mysterious and very difficult to understand.
n.a person, thing, or situation that is mysterious, puzzling, or difficult to interpret. Often carries a connotation of intellectual challenge or intrigue.
“The reason for his sudden departure remains an enigma.”
“Despite years of research, the purpose of the ancient stone circle remains a complete enigma to archaeologists.”
“The reclusive billionaire was a true enigma, rarely appearing in public and never granting interviews to the press that obsessed over his private life.”
Often paired with the verbs 'remain' or 'be'.
“For decades physicists have been puzzling over the enigma of dark matter.”
“In the middle of all this is Theo James’s Eddie, a thick-armed enigma wrapped in a yawn.”
From Latin aenigma (“riddle”), being derived itself from the Ancient Greek verbal noun αἴνιγμα (aínigma, “dark saying, speaking in riddles”).
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.