How can the universe be expanding?

If you were to substitute the word "expanding" for "stretching," it might be easier to understand. Consider a two-dimensional universe that is only made up of length and width. If you were to roll up that surface like you would a piece of paper, it would still be flat but now curved. If you keep rolling it until all the edges join, it will become a sphere, but remember that the sphere's surface is still two-dimensional. If that sphere is then inflated, the surface stretches, growing larger in the two dimensions while actually expanding into the third dimension.

Thus, a three-dimensional universe can also be "inflated" into the fourth dimension, which we refer to as space/time, if a sphere can be blown up into the next dimension similarly to how a balloon would be. In other words, the universe is expanding into the fourth dimension.

Space itself is expanding, which means that the distances between galaxies are getting longer over time. This expansion was first noticed by Edwin Hubble in the 1920s and has been confirmed by numerous observations ever since. The universe is expanding because space is stretching, not because matter is expanding away from a central point. The expansion of space is thought to be caused by dark energy, a mysterious form of energy that permeates all of space.