I was going to title this post "I don't need to know everything" but I think "I don't want to know everything" makes a stronger point. That's why I am going to say it again.
I don't want to know everything.
So don't tell me everything. This does, of course, apply to that dreaded "500 years of history" prologue at the start of your fantasy epic trilogy. But even if you're being a good writer and not giving me large dumps of exposition before the story has even begun, I still don't want to know everything. Not right away, anyway.
You see if your character is in the middle of battle and your narration mentions that the only thing he can do to win is use the special Mattapachinko technique he learned from The Great Sage Charlie Dumbell, that's enough. If you're really into your fighting porn (and I am, so you're not alone) you might describe what the special Mattapachinko technique looks like or why it is effective. But what I don't need to know or even really want to know is who The Great Sage Charlie Dumbell is or how he invented the technique or why or when and what he was wearing at the time. Unless it is really, actually, no-arguments-allowed important to the story, I don't even need to necessarily know how it is your protagonist came to meet The Great Sage Charlie Dumbell and why or how he was taught the special Mattapachinko technique. It's enough to know that these things have names and that they exist.
William Goldman's 'The Princess Bride' is full of this kind of thing.* When Inigo Montoya and The Dread Pirate Roberts are fighting, there's a lot of talk about different fencing techniques and masters and both characters know them inside and out and we're not given any details except that these sword fighting masters and their techniques exist and that's enough. It tells us about the world and the characters and we understand that the more of these great moves a fencer knows, the more learned the fencer must be. That's it. That's all we need to know and, frankly, telling anything can get distracting and disjointing for the story. Best just to leave it alone.
Or say your protagonist is a wizard. He's only a novice Wizard at the Thumpertink Academy. Novice is the lowest rank. Grand MacDonald is the highest rank. There is only ever one Grand MacDonald at any time and when the Grand MacDonald dies, there is a bloody fight to the death for the title. It's held in the cafeteria and anybody may sign up.
Why is the school called Thumpertink Academy? Why are the newbies called Novice and the big guy called the Grand MacDonald? Why must wizards fight to the death for the title? Some of these questions, especially the second one, might have very interesting answers. There could be a lot of rich history involved in all of this. But is it important to the story you are telling? Does it affect your protagonist Wizard in anyway if the history of the Grand MacDonald is revealed? Answer this honestly and if the answer is no, tuck that history into a notebook somewhere and leave it alone.
Nobody needs to know everything about your fictional world, not even you. Nobody needs to know the detailed history, the who and the where and the how of every major event and every significant relic and every secret cult that shaped the destiny of mankind. That doesn't mean you can't have all that and use it in your story. It just means that often enough, it's enough to drop a name or an idea and leave it. The audience can fill in the blanks for themselves or - and this is even better - you can fill in the blanks with later stories! If you give it all away now, what do you have to say in book two?
This is true of any fictional world, whether it's a distant planet, medieval fantasy land or a slightly darker, zombie filled version of Austria.
When you mention something fictional in the world's past or present, be it an event, a place, a person, a particularly popular internet meme, feel free to leave it just one more decorative noun in the picture you're painting for the story. Remember you're not telling a world, you're telling a story. Only tell what me what I need to know for the story.
*'The Princess Bride' also has a lot of exposition dumps but that's part of the style and the humour and plays into his framing device for the story and is another conversation all together.