In order to answer this question, we must first understand what the universe is. The universe is everything that exists; it is all of space and time and all matter and energy. It also includes all forces, such as gravity. So, when we talk about the universe expanding, we are talking about everything in the universe moving away from each other.
Now that we know what the universe is, we can address how it could be expanding into nothing. One theory is that the universe is expanding into a vacuum. A vacuum is an area with no matter or energy in it. So, if the universe is expanding into a vacuum, then it would be expanding into nothing.
Another theory about how the universe could be expanding into nothing has to do with dark energy. Dark energy is a mysterious force that scientists believe makes up 68% of the universe. It seems to be pushing everything apart and causing the expansion of the universe to speed up over time. So, if dark energy continues to make the universe expand faster and faster, eventually everything will become so spread out that there will be nothing left except for empty space.
We don't yet know for sure which of these theories (if either) is correct. But both show how it's possible for the universe to keep expanding even though there's nothing for it to expand into!