When you do not know something, you are "in the dark". Example: " Did you understand question number three?" Answer: "Not at all, I was totally in the dark." To be in the dark about something is to not know it nor understand it. Example: "Did you know that today was her birthday?" Answer: "No, I was in the dark." When you are in the dark, it is as if the lights were off and you could not see. Example: "Why didn't you tell me they were having a party?" Answer: "I was in the dark about it, just like you."