From Longman Dictionary of Contemporary EnglishI don’t knowI don’t knowa)NOT KNOWused to say that you do not have the answer to a question‘When did they arrive?’ ‘I don’t know.’b)SUREused when you are not sure about something‘How old do you think he is?’ ‘Oh, I don’t know – sixty, seventy?’I don’t know what/how/whether etcI don’t know whether to call him.I don’t know thatI don’t know that you need a passport for travelling within the EU.c)DISAGREEused to show that you disagree slightly with what has just been said‘I couldn’t live there.’ ‘Oh, I don’t know. It might not be so bad.’d)ANNOY British English used to show that you are slightly annoyedOh, I don’t know! You’re hopeless! →know
Examples from the Corpus
I don’t know• "Where's the nearest restroom?" "I don't know."• "I couldn't live there.'' "Oh, I don't know. It might not be so bad.''• Oh, I don't know, we could meet at 3 or 4 o'clock.