in the dark
In secret, in concealment, as in This agreement was concluded in the dark. [Early 1600s]
In a state of ignorance, uninformed, as in I was in the dark about their plans. This metaphor often appears in the locution keep someone in the dark, meaning “deliberately keep someone uninformed,” as in They kept me in the dark about their plans. [Late 1600s] For an antonym, see in the know.