As soon as you can imagine and reason about infinity, you might be able to reason about our universe. “Center” makes sense for most bounded things but might be completely meaningless in an unbounded universe.

Example: the number line. You might be tempted to say 0 is at the center but… there are an infinite number of things on either side of any number you choose.

0 is the center of an infinite number line.

Additionally nobody knows if the universe is infinite.

Does that mean nothing is the center of the number line?

Or everywhere is the center of the number line. Some people argue 0 is the center based on symmetry. My original point is that it is non-trivial to reason about infinite spaces. Since we do not know if we live in a bounded space or not, we can’t assume there is one center. Furthermore, even if we do live in a bounded space, there are different ways of measuring center. Eg: center of mass vs geometric center of a volume. Knowing both and comparing them would be pretty interesting!

There's nothing special about the nozzle; it's just another point. The nozzle isn't the center. But if you can't get past that, imagine a balloon without a nozzle, with a canister of compressed gas inside, and the compressed gas is leaking out of the canister.

I mentioned a perfect geometric sphere that is expanding has no center on that surface.

I also stated that no real world analog of a perfect expanding sphere exists. Because we can't imagine an analog it's much more likely that the universe DOES have a center similar to the nozzle.

Try to imagine a real world analog to the expanding geometric sphere without a point of discontinuity. You can't. And because you can't maybe it doesn't exist. Maybe it does. But not being able to imagine a real world analog is slight evidence that there there is a center.