When a quantum wave function decoheres, it doesn’t precisely collapse into a singular state. Instead, it transitions into a more restricted superposition that resonates with the environmental conditions, effectively integrating into a more intricate quantum system.

This implies that not only individual particles are in a state of superposition, but the entire universe exists in such a state. However, this superposition is so extensively constrained by interactions and entanglements that, at a macroscopic scale, the universe behaves according to classical mechanics.

ELI5: Imagine the universe is like a giant game of pretend where everything can be in many stories at once—like a cat being both awake and asleep in its adventure, or a ball that’s both rolling and still. But, as soon as we peek to see what’s happening in the game, everything picks one story to stick to where the cat is either just awake or just asleep, and the ball is either rolling or not. But, the game is so big and involves so many things that, most of the time, it seems like everything is following simple rules, like in a regular game, even though underneath, it’s still playing pretend with all the possibilities.