Yes! Because all gathered evidence that we have points to a single cohesive conclusion - that all matter is contained in the universe, the space between that matter is just expanding. We have zero evidence to suggest that there is anything outside of what we refer to as “the universe.”

There are certainly things outside of our light cone, which, for all practical purposes, are outside of our universe. If you imagine a flash of light from a specific place at a specific time, and that light traveling (say theoretically infinitely) outward in all directions, there will always be something outside of that realm of possible influence. Like if something is 100 light years away and it explodes the moment that I was born, and I live to be 99, the effects of that event will never reach me because nothing can travel faster than the speed of light.