The power of "I don't know"
When you don't know something, admit it. Honestly, it will be OK. The alternatives will damage your credibility, and hamper your reputation.
In our product team, I have a reputation of being the sponge of institutional knowledge, the person that everyone asks when they need to know something. I am not sure how I got that reputation, but it just ain't true.
Sure, I have a lot of tribal knowledge, and a large library of links to arcane and dusty corners of our institutional knowledge. But that isn't the whole explanation. It isn't why I get a few pings from left field each week from people around the organization.
I am going to let out a secret. I really don't know everything. But, when I don't know, I can usually find out. I am resourceful that way.
How I cope?
It can be uncomfortable to not know the answer to a question that you ought to know. Many people will hem and haw, or bluster, or fake it. Hell, isn't there a credo "fake it until you make it"?
When I am stumped - and that is not that rare of an occurrence - I just admit it straight out.
"I don't know"
It rolls off the tongue. It is the truth. It answers the question. But, if I stopped there, I would not be me. Part of my genes is to be inquisitive. It is why I studied Physics in college. It is why I am constantly reading and expanding my universe.
And when someone asks me a question, and I don't know, I make it my business to find the answer. To run down the information. To get to an answer (or, if it really has no answer, to prove it).
Why "I Don't Know" is powerful?
This is simple. Because it builds trust. Most people I work with, or have worked with, when they are confronted with an ask that they can't respond to will ignore it or deflect. Email makes this easy. Just "forget" about it. Once, is not a problem. But, it is almost never a single event. A few times of people asking you questions and you go radio silent, sure, you don't 'fake' it, but you gain a reputation of not being responsive. And in modern workplaces, that is very much a bad place to be.
And that is far far worse than just admitting that you don't know.
Nobody really expects you to know everything. That's impossible. Hell, Einstein reportedly never remembered his phone number, reasoning that it was at hand in a phone book, why should you remember it.
But, if you want to be a valued colleague or coworker, engage, admit it when you really don't know, and if you are inclined like I am, run down the right answer.
Your reputation, and your career will thank you.
Coda
I really really hate the "fake it 'til you make it" mindset. If you aren't going to put in the effort, you shouldn't get a pass.
Perhaps part of my drive to get to an answer or disprove it is related to my imposter syndrome. Regardless, it is in my DNA to chase the thread.