I don't know.

Meaning: I do not know. / I disagree.

Extra info: In very casual conversation, "I don't know" sounds like "I dunno." In very casual texts and emails, many people write "I dunno" instead of "I don't know," but never in formal emails.

Examples:

-Do you like dancing?

-I don't know, I never tried it.

-Let's have natto for lunch.

-I don't know...

(True meaning: I don't want to.)

-The Beatles are the best band of all time.

-I don't know about that.

(True meaning: I disagree.)

-Should we visit London or Paris?

-I don't know...

(True meaning: I can't decide.)

-Who broke the computer?!

-I don't know!

(Strong "I", true meaning: Someone else, not me!)

-What should we do today?

-I don't know, what do you want to do?

(True meaning: I have no ideas.)