The relief of saying "I don't know"
What is not necessary is redundant. Saying "I don’t know" is not a negative admission of ignorance, but an assertive refusal to engage with a thread irrelevant to your life.
When we think about FOMO - which, incidentally, you've just experienced in case you are anxious about not knowing this term - we imagine missing the festival your friends recently were in, failing to taste the feeling of owning huge wealth or worrying that you possibly are wasting your life whereas the people around seem to live the dream. But FOMO can also appear as a subtle anxiety that you must have an opinion on whatver topic people around you find interesting.
Consider this specific example.
At the family reunion, your uncle argues about a very specific political topic and you can sense a growing urge to just add a little comment. It seems like you've got a crucial thing to say, a seemingly conversation turning point, that happens to be an opinion you've listened recently. You take the courage to speak your mind and it feels like entering a war. You are no longer a neutral state and start to deploy your own army, trading fire with others.
But at some point the opponent asks an unexpected question. Suddenly you are touched with anxiety as your mind is now blank. This time you don't have this robotic answer which is just someone else's digest of the topic instead of the outcome of your own studying. Your argumentation falls apart, but it is hard to retreat though, as saying 'I don't know' would be an unpleasant lost battle. The bigger problem though is that what you thought was 'your' understanding was just parroting opinions. And now you realize it, but your ego just hasn't accepted it yet.
That's just one very specific example you might have experienced.
But more generally, you might believe that unless you are up to date - with the news, the TV show your colleagues happen to binge or whatever else - you're missing out on life itself. But you are not. Apart from trying to become the best you can in your profession and caring for the people who truly matter to you, the rest of the topics you keep up with just to stay ‘up to date` are frankly arbitrary. So when someone asks you a question irrelevant to your life and you happen to not know, there is no need to stress. Just say, in alignment with the truth: "I don't know".
And through this seemingly ignorant statement, the freedom of non-engagement in the project irrelevant to your life brings a gentle sense of relief. And when you train in it, patiently enduring the discomfort of being 'ignorant' in your circle, you eventually will stop putting effort in the topics just to be liked. Instead you fully focus on the things that are factually important for your career or personal well-being. Choosing to admit 'ignorance' in your community might not feel good in the short term, but over time your life will start to revolve around fewer but solely meaningful pursuits. You will finally be able to rise in what truly matters, earning the respect for your depth and possibly becoming the leader in specific area, instead of being a reactionary follower, constantly desperate not to appear ignorant through borrowed opinions.
What is not necessary, is redundant. Ponder on which threads are worth your continued attention, and which ones you can let go of, so you can conciously stay peacefully ignorant.