The concept of the expansion of the universe can be a bit counterintuitive to grasp, especially when we think of the universe as everything that exists. It is important to understand that when scientists say the universe is expanding, they mean that the distances between galaxies and other cosmic structures are increasing over time.
The common analogy used to explain the expansion of the universe is to imagine the surface of an inflating balloon. As the balloon inflates, all points on its surface move away from each other, just as galaxies move away from each other as the universe expands. However, it's important to note that this analogy is meant to illustrate the concept of expansion and not to describe the actual geometry of the universe.
In the case of the universe, it is not expanding into anything in the way that we typically understand it. The expansion of the universe refers to the stretching of space itself. It is the fabric of space itself that is expanding, carrying galaxies and other structures along with it. There is no outer "boundary" or region outside the universe that it is expanding into.
It's important to note that the idea of the universe's expansion is based on observations and measurements of the cosmic microwave background radiation, the redshift of distant galaxies, and other astronomical data. These observations suggest that the universe has been expanding since its early moments following the Big Bang. However, the exact nature and mechanisms driving this expansion are still active areas of scientific research.