What is the word to know / understand something / someone's existence. Realization seems close. But i want a word with the essence "It's shocking that I didn't know this till now."
A closer variation on this would be the word epiphany. When we first learn something it is usually by rote and we do not full understand the processes behind what we learn. Upon further learning and understanding we arrive at what I call the eureka moment. This is where you say "AHA I understand why it is?" This moment is called an epiphany (apologies to the definition purists)
"Enlightenment" describes that pretty well, as "enlighten" means literally "to remove the dimness or blindness". Connotes that you can see more clearly, in a figurative sense, and recognize the existence of something you figuratively could not see before.
"To have a revelation" is another good way to describe that. Literally "re-" (opposite of) + "velare" (veil, cover), so in a sense meaning to uncover something that always existed, that you had never seen before. ("I had a revelation" would be a good was to express the sentiment you referred to.)
Then there is also "become aware," "become cognizant," etc.