The universe is full of mystery as it contains immeasurable creation, history and event—the reason why it's immeasurable is because of how rapidly the universe continuously expanding over the time.
The universe isn't a physical world, but it was a complex algorithm where it holds many cause and reason—if place [undefinable size] that are called as the sea contained many water since we wouldn't be able to count how many it is exactly, then the water much be contained millions if not trillions or more atoms in microscope size, but if we zoom it more then we would be able to see countless quarks inside a massive size of a sea.
The universe is not things or something that could be defined scientifically as it flowing like a water in absurd way, like a bubbly wubbly rivers that could be in any shapes—basically the universe itself is a 3-Dimensional temporal+1-Dimensional spatial, the universe is a fourth-dimensional plane where it helds countless causality and probabilities. If something have the power over a single universe, that entity have the power to change the course of history, event and the concept of probability itself inside of the universe. Imagine how powerful the entity would be if it could manipulate over the entire infinite universes—the probability is undefinable.
The fourth-dimension isn't really difficult to understand, it exist as spatial geometrical size of causality and idea—it holds many possible worlds that contained many branches with incalculable amounts of universes. The reality itself is part of every dimensions and every dimensions is part of every reality, if a 3-Dimensional beings perceive something with lower dimensionality than them. Does that makes them sees it as a mere fiction, the answer is. No, but it's just if a singular dimension where other higher realities with infinite dimensions only does exist—since this entire creation contained many aspect of undefined realities, dimensions, universes and many more. The answer would be, yes since each dimensions would be undoubtedly superior than lower dimensions. That makes if a single box with infinite dimensions in it, then anything lower it—but if the box have more stacked of boxes like infinite sets of boxes, then each box transcends the lower boxes that contained the entire dimensions.
Although it sounds intriguing, remember that everything is basically a small specks of dust compared to the universe. Universe is a small speck of dust compared to a 4-dimensional plane of existence—it keeps repeating like a cycle, it endlessly repeatedly transcending and transcends everything lower it and the lower transcends the higher dimensions, yet the higher is transcendental to everything below it.
If the world itself is nothing to an entity that perceive all creation as nothing but mere illusion that it created, and if a person acknowledge about it—will the person accepting the reality or will the person denying from accepting the truth. Keep denying something isn't always the right option, but it is sometimes—that's how the universe is, it is gigantic yet it is small. It is real yet it feels false, the only thing that is real for them would be nothing as they have the knowledge that everyone do not know.
The universe have many variants. For example—Parallel world, Alternative world, Copied world, Mirror world. What is parallel world? What is alternative world, copied world and mirror world? It's simple to explain. First, Parallel world is the same exact world, exact time, exact events but it have many different individual and invention. Secondly for alternative world, it is different world, different time, different events. Thirdly for copied world, it's a copy version of the two worlds. It could have different laws and physics for example, a single math one plus two is equal to three—but in this world it could be any some sorts of numbers. On the other hand, the mirror world is a world inside of a mirror. The vastness is immeasurable and is very identical