This theory makes no such assumption. One could say that it is simpler so it would fit Occams razor. Even further, it predicts that you can't find dark matter at all (although this can't be proven, so there's that) as much as special relativity predicts that you can't find "aether".

I think, "simpler" would be compelling enough, but I can't judge whether it is the case really.

On another note, does anybody know whether "inflation" makes testable predictions? From my little knowledge it also explains only already known phenomena. Nevertheless it is a widely accepted theory.