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.