A common phrase used to admit you lack information or certainty about something. It can politely decline to guess, express hesitation, or soften disagreement.
A: I don't know about back. Source: Internet
Ali added, "'I don't know, we'll see. Source: Internet
"And also there's a new homologation this year for the overalls, so they are thicker, and the underwear is thicker, so I don't know how much effect that has." Source: Internet
"And he's denied it, I don't know whether he said it or not, but even the possibility of that is just horrible." Source: Internet
And I don't know if that was a byproduct of how we made it, but I also wrote a lot of the songs pretty quickly, one right after another. Source: Internet
And I don't know why, but you just become hyper aware of your body at that point. Source: Internet