Dark matter isn't a solution, it's a quantification of the unknown. It's more nuanced than just "I don't know", but it's not far from it. I think you're wrong that science is bad at those words, it's just that's it all plays out a little more subtly than that. There are lots of things we DO know about this unknown phenomenon that we've named dark matter, and we have lots of theories, but everyone will admit it's still a bit of a mystery.