I don't know, but...
It’s a fine thing to say. Admit we’re not sure, we don’t have all the answers, we don’t know.
This can feel like weakness, but in reality it suggests strength.
You’re confident enough to admit when you’re missing some pieces. The followup, then, is a response that acknowledges the gap, while suggesting you can take the steps to find out.
Being curious, open to new information, offers opportunity for others to share what they know.
The person who must always have the first word, says “I already know that”, misses out on the chance to learn something new.