I'm sorry, but scientists rarely talk about dark anything as if it is an unknown. They use it all of the time as a central component of their explanations.
It's as baseless as the ether of the 19th century and as ubiquitous in its invocation.
That's not entirely fair, because treating it as "matter" gives a quite good mathematical fit for current observations so far, while the first experimental observation of light speed ever made immediately falsified the ether hypothesis.
It took a while even after the experiment was repeated a couple times. And even after the experiment only very few could abandon the idea of ether, and only after abandon the idea of ether, they see the new physics. Now with dark matter, we have no experimental idea and it is out-of-reach. It is there to protect the consistency (cover up the inconsistency) of our current understanding -- familiar?
e: I'm agreeing with you that putting "matter" in its name is fairly misleading. Fortunately, that doesn't confuse physicists.