In our daily life, there is at some stage that we’re going to use the expression, “I don’t know” indeed, so it is as a variety is the spice of life, using different ways to say the word, “I don’t know” would make your conversation far more interesting and natural.

This word is used informally and in spoken language, especially by teenagers who don’t want to answer their parents!
e.g. ‘When do you have to submit this essay?’ ‘Dunno. I can’t remember.’

These expressions are used when you have no information and you cannot guess the answer to a question.
e.g. ‘What time does the film start?’ ‘I have no idea. Why don’t you call the cinema?’
e.g. ‘Would you by any chance know where St James’s Street is?’ ‘I haven’t a clue. Sorry.’
e.g. ‘Could somebody please explain how this happened?’ ‘I haven’t the faintest idea.’

These expressions are used when you do not know something and you feel annoyed that someone is asking you about it.
e.g. ‘Who left this mess on the table?’ ‘How should I know? I’ve only just come home.’
e.g. ‘What time did she say she was catching her train? ’‘Don’t ask me. I’m always the last to know.’
e.g. ‘Why didn’t he ask you for the books?’ ‘Look at me. I’m not one who likes reading.’

These are used for saying that you don’t know something because it is impossible for anyone to know it.
e.g. ‘Will they ever find the missing aircraft?’ ‘Who knows? The search has been going on for such a long time now.’
e.g. ‘How the situation will develop from here is anyone’s guess.’

In this way is using for something to say that you know as little as the person about something who asked you about it.
e.g. ‘Do you think we will be able to book the restaurant for Friday?’ ‘Your guess is as good as mine.’

We use this to say that something may be true, but you do not have enough information to know whether it is or not.
e.g. ‘Has Clive left the company? I haven’t seen him for ages.’ ‘Not as far as I know, but I haven’t seen him recently either.’

This is a rather informal expression to say that you do not know or understand something. “It” is often left out.