My high school physics prof had the best answer to "What is beyond the universe?" --> "Surely if we ever find something outside of the universe, we will be smart enough to call that the universe too"
He was referencing the idea that universe contains everything by definition.
Universe is our local observable space-time with the earliest event we can detect being the big bang.
So if we observe something beyond what is currently observable, would that not become observable and thus part of the universe?
Transitivit would say yes. If A implies B and B implies C then A implies C. That is, if you can observe A and that implies B, then you can observe B. Which makes it observable, no?
For now we're stuck in our universe, we know it's expanding (in something, potentially). So we preemptively give the name "cosmos" to EVERYTHING. It's entirely possible that cosmos is equal to universe. But it's also possible there are other universes out there, or something entirely different.