If you are "in the dark," you do not understand something. The image is of a person fumbling around in a black space, trying to find something. The antonym of this is "to see the light," which means to suddenly understand. If you just don't know what something means, you could say "I'm in the dark on that subject, could you explain it to me?"
This means that you don't understand.
It's not an idiom because you can figure it out. It's a sarcastic way of saying something is not clear - it's as dark as mud.
Whistling in the dark means you really don't know what's going on are trying to comfort yourself.
It's not really an idiom. It means "what are you thinking about."
RFP is not an idiom. It's an abbreviation.
It is not an idiom, it means your nose is itching.
idiom means expression like a page in a book
It's not an idiom. It means the tip of your nostril.
"Sieve" is not an idiom. See the related link.
Phil doesn't know what's going on
This is not an idiom. It is a measurement. $100,000 is how you write it in numbers.
Simply its mean a bully.