Use when you truly know nothing about the subject and want to communicate that.

Expressing Ignorance

It suggests that not only do you lack information, but you also cannot make a guess.

Admitting Uncertainty

The phrase adds strong emphasis on your lack of knowledge about a specific detail.

You can use it when a fact is unexpected and surprising to you.

Shows you’ve possibly underestimated something, often with a tone of caution or warning.

When directed at others, it might point out their lack of awareness about something important.