Well, attributing it to dark matter is easy. It's not really explaining anything though, until you can say what dark matter is and why it's there. At this point it's basically just an abstract parameter that we assume, without any direct evidence, is in fact some kind of matter and not, well, something else. Maybe spacetime just have different geometry in different places? Maybe the assumption that it would be flat in the absence of matter is wrong? It could very well be that the neutral shape of spacetime in some places is more or less curved than we currently assume.
Then there are fun ideas like large masses permanently deforming spacetime. Going with the famous rubber mat analogy, what if the mat gets permanently stretched and even once the mass that stretched it is dispersed the sag remains?
That's of course assuming there even is such a thing as spacetime. Which, btw, sounds like a reasonable assumption. However, just because the math works doesn't mean it's a physical reality, it merely points to it.
That's the problem. It lumps unexplained phenomena into a luminiferous aether 2.0. The most likely explanation is our model of gravity isn't yet good enough and so perhaps RelMOND is worth a look.
Hence: dark matter - because the effect we observe seems to behave that way. MOND is distinct because it declares what we're seeing is not a bunch of entities with that character.
It's worth noting we also talk about a lot of things like this even though strictly speaking they're not: i.e. electron-holes in semiconductors can be perfectly well modeled as their own form of matter - they can't just leave the semiconductor environment. But they do have momentum and position.
Dark matter could literally be the same type of effect, but if you're a wrinkle in space-time then it doesn't matter unless that description has some sort of experimental reality, and we currently do not observe it to do so. It is not like physicists would not rush to test any experimental formulation of dark matter as such that was accessible with current or near-future instruments.
There are rogue blackholes, maybe they leave spacetime deformations behind them that could be observed?
I wonder if that can explain some of the large scale filament type structures we see.
It doesn't mention the Bullet Cluster though.
We see some galaxies whose gravity is near the baryonic prediction and others where it is not.
- explaining this with dark matter is easy: some galaxies have a lot of dark matter and some have little. - explaining this with Mond seems impossible. If there is some gravity law we don't understand, how can it have different effects on different galaxies?
- how do we explain the famous bullet cluster finding?