unclear, obscure, dark
La cause de l'accident est encore obscure.
The cause of the accident is still unclear.
L'origine du mot est obscure.
The origin of the word is obscure.
Sa raison de ne pas y aller est encore obscure.
His reason for not going is still unclear.
Quelques jours, dans cette cave obscure ?
A few days in this dark cellar?
— Psychose (1960)