How do we know the universe is not of constant size?

>inb4 because it is expanding
Yes but couldn't that just mean that the space time continuum is stretching and the edges are being "trimmed off" because of a loss of energy?
So imagine pair production could occur at the edge of a static universe, one of the subatomic particles would for all intents and purposes be lost, ergo we have lost energy, ergo the space time continuum stretches while the universe's size remains the same, right?
Maybe there's another piece of evidence I'm missing but Hubble's shit just seems a little too weak to me.