I have always wondered why we consider that galaxies are drifting away and not just contracting at a particle level so that locally there is no visible contraction. I mean, if gravity consumes energy, that would make sense that all particles shrink simultaneously, no?
An expanding universe can be formulated as a solution to Einsteins theory of general relativity, which remains very successful until today. At the same time I don't know about any scientific theory of shrinking matter.
I have always wondered why we consider that galaxies are drifting away and not just contracting at a particle level
Two good reasons:
1) General Relativity, the "formula" for gravity, is highly unlikely to result in a stable universe, or "flat". It is much (infinitely?) more likely that it is expanding or contracting. The fact that we seemingly observe it expanding strongly suggests that we are indeed seeing it actually expanding.
2) There is no analogous physics in Quantum Mechanics, the "formula" for matter, that suggests the distance between particles changes over time. This could happen, but it would require changes to fundamental values that we've measured to the 25th decimal place and do not appear to be changing. It would also leave a mark in history if these changed, which we could notice through telescopes, and we see no sign of this.
So basically we expect to see this happening because of (1) and don't expect it in (2) and that's enough for most people :-)