Does the statement "I don't know" make you vaguely uncomfortable? Do you try to avoid using it? Have you ever been tempted to invent an answer rather than admit that you didn't know?
"I don't know" can be an uncomfortable truth, but it is also a necessary one. It is found at every limit of human understanding, beyond which is all too often someone's lie.
"I don't know" is not an excuse for someone to fill a gap in current understanding with a fanciful tale that happens to be appealing or convienent. Passing such a story off as the truth impedes knowledge and stifles curiosity.
"I don't know" does not have to remain a permanent condition. If we are honest about what remains to be discovered, we can more easily search for answers. We cannot cast light into the dark shadows of our ignorance if we aren't honest about where they are.
I think any shame we feel over this should be reserved for pretending to know. Being honest about not knowing is just the first step towards real understanding, since what we really mean to say is "I don't know, yet."