I would call 'dark matter' a description rather than an explanation. It seems to effect gravity. So it has mass. So it's 'matter'. But other than that it is unobserved, and so 'dark'. A+B = a very short but accurate description of an observation, rather than an explanation of why that observation happens.
Well, it's a bit of both. It's an explanation in that it explains the observational discrepancies in terms of matter that's difficult to observe, rather than, say, different laws of physics or extra dimensions or whatever. But it's also just a description in that it just means some sort of matter with mass that's hard to see.
