Using different ways to say “I don’t know” would make your conversation far more interesting and natural.
• Dunno (informal)
Where do you put the car keys? – Dunno. I can’t remember.
• Beats me (informal)
Who was the 35th president of the United States? – Beats me! Why don’t you Google it?
• I have no idea
I have no idea why she did that.
• I haven’t a clue (OR I haven’t got a clue)
Do you know the answer to that question? – I have no clue.
• I haven’t the faintest idea
Could somebody explain how this happened? – I haven’t got the faintest idea.
• How should I know? (when you feel annoyed that someone is asking you something)
Who ate the leftover pizza in the fridge? – How should I know? I just got home.
• Don’t ask me (I’m always the last to know)
• Who knows? (use to say that you don’t know something because it is impossible for anyone to know it)
Will they ever find the missing plane? – Who knows? The search has been going on for over three months now.
• Your guess is as good as mine (use this when you know as little about something as the person who asked you about it)
• Not as far as I know (use this to say something may be true, but you don’t have enough information)
Is Julie and Barry going for a divorce? – Not as far as I know, but Julie told me that Barry has moved out.