ELI5: Explain Like I'm 5

what is cosmological inflation?

Cosmological inflation is a theory that explains how the universe got bigger really quickly after it was created. It works like this: During its first tiny fraction of a second, the universe expanded much faster than it does today. This quick expansion pushed the universe further and further apart, and it's been growing ever since. Think of it like someone blowing up a balloon - the air expands quickly and pushes the rubber further and further out. Inflation works in the same way, but instead of a rubber balloon, it's the universe that's getting bigger.