science

I'm a layman, too, so take everything with a grain of salt.

As for evidence, if I both understood and remember correctly, the maximum distance we can actually see something (Hubble radius) just happens to align quite nicely with the Schwarzschild radius, a parameter based on the mass of a black hole, which correlates to its radius. They have to be identical for this theory to be true. Them almost being so could be a coincidence, though.

In addition, from our perspective, there's no real difference between an expanding universe and one with shrinking particles. If the planck length actually shrinks, to us, it will seem like everything else will move away. Within the last 100 years, multiple people created some models for that, proving how it could work while leaving physics as we observe them intact.

A proof could be found by observing a white hole, the opposite of a black hole. A space you cannot possibly enter, ejecting energy. Think of it as the stuff entering the black hole from the outside, as oberserved from the inside. They are just a theory for now.

Once again, I've got not actual clue and you might want to dive into that rabbit hole yourself. It's fun in here.