He didn't actually 'observe' dark matter. What he did, was notice that his equations weren't balancing out the way he thought they should. So, rather than say "perhaps we need to figure out what's wrong with our current understanding of physics and cosmology, he invented something to balance things out again. I suspect we actually know less about the way things really work on both the large and small scales than we think we do.
That is a possibility. When cosmologists use “dark” to describe dark matter or dark energy, it doesn’t just mean we can’t see it. It also means we don’t know what it is. It could be that there is something missing in our own physics, or there is something actually out there that we can’t see. Either way, it is “dark”.