I mean, we need to have a convenient term for a phenomenon even if we don't understand it. What's the alternative? "The unnamed phenomenon were some astronomic bodies behave as though a vast quantity of matter were affecting them but that matter can't be otherwise detected or identified"?

In medicine, they often call it a "syndrome". A collection of symptoms that together form a known pattern that is often closely associated with a disease or disorder but has no understood cause or origin (although some syndromes continue to be called that after they are better understood).

How do you think we arrived at terms like "planets"? We noticed that some stars in the sky moved around while the rest didn't, and we called them "wandering stars". We still call them "atoms" even though that word means "indivisible".

Weather is full of such names: rain, thunder, lightning, wind, morning glory, dew, tornado, rainbow, etc. All these words for material phenomenon existed long before we knew how they worked.

So what's wrong with "dark matter" as a name?