You ever wonder about the wide universe of things out there that the human brain is simply incapable of comprehending?
Sometimes I wonder what it's like to be a dog or really any other non-human animal, and I think about how simple their understanding of the world around them must be given their lack of ability for much abstract thought. We take for granted all the ideas that are extremely obvious to us that other animals will absolutely never come close to understanding.
There are no creatures on Earth smarter than humans. Elephants, cetaceans, and non-human apes may come closer than we perhaps realize, but there are no creatures that absolutely crush us on this front, we have yet to be humbled. I feel like this gives rise to the common perception that human intelligence is infinite, that we are destined to understand every truth about the universe given a long enough timeline and enough effort.
That's a comforting thing to believe, but I don't think it makes sense when there's no reason at all to think we're the apex of intelligent lifeforms in this universe. It not impossible that there are alien species out there that would view our understanding of the world in much the same way we view that of other animals.
In that case, the implication is that there are any number of shockingly huge and fundamental revelations about the universe sitting right under our noses that we simply lack the ability to even perceive much less understand. This to me can be a bit of an unsettling line of thought to explore. It's the fear of the unknown with heavy reinforcement from the likely possibility that the unknown will always exist, often just beyond our reach.
"We live on a placid island of ignorance in the midst of black seas of infinity, and it was not meant that we should voyage far. Also I am racist against the Welsh." ~HP Lovecraft
The last bit is the most important. Tell me that Llanfairpwllgwyngyllgogerychwy rndrobwllllantysiliogogogoch isn't the name of an ancient city inhabited by extradimensional horrors. You can't.
The example I always use for this is what's outside the universe: nothing.
Its not just missing physical things, atoms and all that, it's missing the space for those physical things to not exist in, and even the passage of time for that space to not exist in. You can describe it, but imagining it is impossible, because there's literally nothing to imagine.