I am no big fan of Donald "let's invade Iraq for breakfast because I'm a white alpha male" Rumsfeld, but I do appreciate the wisdom of his Unknown Unknowns speech.
There are many things I'm scared of and numerous things I regret, but if we narrow these to that last of all deliberations - the things I'm going to be pissed off with at my death bed, assuming I have the time and capacity to do so - I am voting for these Unknown Unknowns taking centre stage.
I know it is certain there are tons of movies out there I would like - a lot - but will never get to watch because I am unaware of their existence. It is a statistical certainty that the book that would earn my vote for "best written words ever" is a book I am completely unfamiliar with. And I know for sure there are things I would love to know about this world but will never learn due to the fact they will be discovered after I'm already gone - stuff like how consciousness works, the discovery of extraterrestrial life, and much much more. FTL, perhaps?
Over long periods of time, science and human progress in general should have little boundaries; yet I will only catch a small glimpse of those.
I can try and comfort myself by telling me I'm trying, doing my best, to find that best book, to watch that great film, and to learn more about this interesting universe that we live in. But I know all too well that, at the end, I will look back at myself and conclude I just didn't try hard enough.