Dark matter makes the prediction that there is some kind of matter which makes no or little interaction with ordinary matter. None of those has been found so far. This makes it an assumption.
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.
This is incorrect. Scientific theories explain observed phenomena, and must be falsifiable, testable, and tested against the best available observations. However there is no requirement that these observations happen in the future. Evolution is a good counter example to your assumption. Anyway, in cosmology we can't wait for the future, its too far away.
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.