In the dark (about something)

In the dark(about something): Not knowing very much about something, because other people are keeping it secret from you [ macmillan]
In the dark: to not know about something that other people know about. [Cambridge]

In the dark: "Não sabe de nada".

• We are still very much in the dark about how the money was lost.[Nós ainda não sabemos muito bem como o dinheiro foi perdido]
• You’ve kept us totally in the dark about what happened that night.[Você nos manteve totalmente no escuro sobre o que aconteceu naquela noite.]
• You're totally in the dark, Mr Gullible![Sabe de nada, inocente!]

Fonte: www.macmillandictionary.com; www.englishexpert.com.