If it explains everything then why are experiments still ongoing? What is left to explain?
In contrast, there are phenomenon which we know standard particle physics and quantum mechanics gives the wrong answere (despite getting so much else right).
...except, isn't it true that the galaxy that contains the earth wouldn't exist were it not for dark matter?
If you do not possess that curiously then so be it, but others are still motivated by it.
And then of course as there are complex emergent effects of multiparticle systems that are difficult to calculate accurately.