Saying “I don’t know” is hard. It’s an admission of a shortcoming, a lack. It says, “I lack knowledge” or “I lack understanding”. Not saying it is worse though. Making up some sort of answer says the same thing, shows the same lack, but it includes a lie; an inability to admit reality or be perceived as inadequate. News flash: we all are inadequate (on a way more frequent basis than we’d like to admit). The difference is you choose to ignore reality. A lack of knowledge is something that could possibly be remedied. But lying about realities, that’s intrinsic, something fundamental and not something that you can easily fix. Oh, and the person you’re talking to will know it - they just might be polite enough not to say anything. Do yourself a favor; just say “I don’t know” and we can both go from there.