Think about a bubble of air, exhaled by a diver deep under the ocean. It is compressed by the pressure of the water around it. It starts out small. All the oxygen, nitrogen, and carbon dioxide molecules are close together. But as it rises, the pressure lessens. The bubble gets bigger, and as it does so, the space between individual molecules increases. Nothing fills those spaces between the molecules, it just gets bigger. But the distance between atoms in those molecules does not change.

Our universe is a sort of bubble of space-time, expanding into a higher-dimensional space or a quantum vacuum. Like the atoms in our air molecules, local distances stay the same. But looking further away, we can see that things far away always get further away, wherever we look. This is the expansion of the universe.

Unlike a bubble under water, our universe does not have a edge that can be detected from within it. No matter where you look, it all looks the same. Space-time might be a closed sphere, so that if you travelled in a straight line, you would end up where you started. It could be a torus (like a donut) where you might travel in a straight line and never return to where you started. There are many other curved options for our universe. Some are closed and some are not. We don't know, and we may never know.