Re: It's quite a small object
"Dark matter is the luminiferous aether of our age. It was invented to prop up existing theories"
Occam's razor.
There's an effect that acts like an unseen mass affecting galaxy's rotation. It could be an undiscovered interaction between mass and space, or matter/energy/space, or it could be an unforeseen consequence of known laws, or it actually could be an undetected mass.
Simplest explanation is undetected mass, so most research tends in the direction of figuring out what it is.
Next simplest is unforeseen consequence of known laws, so if the undetected mass research finds nothing, that's what gets looked at next.
If that doesn't pan out, then we're left with undiscovered interaction(s), which will take a paradigm shift to uncover.
So we're testing for the easiest and most likely answer first, then the next likeliest, until we're all just standing around scratching our heads saying "WTF?" until someone has a "Eureka! moment", then we all say, "Duh, how obvious! How did we miss that!?"
That's the way it's worked with science since the beginning