ways of saying ‘I don’t know’ : 
1) Dunno:
used informally, in spoken language.


-A: ‘Are you cAoming out later?’ 
-B: ‘Dunno. Depends on the weather.’
2) How should I know?/Don’t ask me/Search me: 
used when you do not know something and you feel annoyed that someone is asking you about it.


-A: ‘Who left this rubbish all over the table?’ 
-B: ‘How should I know? I’ve only just come home.’

-A:‘Why didn’t he call me himself?’ 

-B: ‘Don’t ask me. I’m only the messenger.’

-A: ‘Why didn’t he say he wasn’t coming?’ 

-B: ‘Search me. He never tells me anything.’
3) Who knows?/It’s anyone’s guess: 

used for saying that you don’t know something because it is impossible for anyone to know it.


-A: ‘When will this situation ever be resolved?’ 
-B: ‘Who knows? It’s been going on for so long now.’

-A: ‘How the situation will develop from here 

-B: is anyone’s guess.’
4) Not as far as I know: 
used for saying that something may be true, but you do not have enough information to know whether it is or not.

-A: ‘Has James left the company? I haven’t seen him for ages.’ 
-B: ‘Not as far as I know, but I haven’t seen him recently either.’
5) It beats me: 

used for saying that you do not know or understand something.

-A: ‘Why did he do such a stupid thing?’ 
-B: ‘It beats me.’