Well, that is sorta kinda how it worked. In a nicely behaved keplarian galaxy, stars somewhat closer toward the center, would move faster and those farther out would move slower, like our planets, and you predict their motion, using Kepler’s laws. But, they didn’t. They moved more like the galaxy was something like ten times as massive than the stuff they could measure. So, it was theorized that there was a whole bunch of mass they couldn’t see.
You can’t see it, touch it, taste it, or put a voltmeter on it. And it’s not neutrinos, they know a little bit about them, and they ain’t it.
So, they can’t observe anything except it’s gravitational effect.
Now they discover some new property. In the early days of the Solar System, bodies moved by their orbital motion, but, the system was pretty thick with dust and gas, enough to have an effect on the motions of these bodies. The dust particles don’t orbit in the same fashion as larger bodies, they were a bit slower, and they also created a drag on the bodies. It seems that dark matter does something similar where it’s apparently really thick.
So, like every new concept, the current model of how the universe works will be adjusted to accommodate the new observations. That’s how science works and how our knowledge improves. No doubt we will discover more amazing things about dark matter and dark energy in the years to come.
Or, we’ll discover they are really something else we never knew about.
That’s what keeps science interesting.