However, it's important to note that this phrase is quite informal and used in casual situations. You would be unlikely to use it in a formal setting.
Here are some ways to express "You don't know" in a more formal way:
* Vous ne savez pas (formal "you" singular)
* Vous ne savez pas (formal "you" plural)
* Vous n'êtes pas au courant (You are not aware)
Let me know if you have any other French phrases you'd like help with!