Well, in this case they're not just inventing nonsense to fix one little thing. There are a lot of various pieces of evidence, all pointing to this fix. If you want to patch up a lot of little pieces of evidence, each with it's own magic, that would be even more "un-parsimonious".
The evidence for dark matter is pretty solid.
As this [1] article states, while giving 5 independent reasons scientists think dark matter exists, "No other idea explains even two of these".
Wikipedia lists eleven different places it shows up.
The research literature has more. And AFAIK there is no other unified (or even close to unified) explanation for all these observational pieces of evidence.
[1] https://medium.com/starts-with-a-bang/five-reasons-we-think-...