[Aachen]

It's not that there is a shifting boundary where the universe ends, but what is meant with universe expansion is (per my understanding) that objects grow further apart over time without experiencing acceleration or any force acting upon them. It's like drawing a map on grid paper and then increasing the grid size without adjusting the scale: all distances become larger

This stretching includes any photons traveling across the grid, and the stretching causes them to expand. Longer wavelengths are more red, giving us the word redshift for stretched light rays. That's one of the ways we measure the expansion: see how much redder the light is as you look at more and more distant objects