I think dark matter is a synonym for "undetectable-through-normal-means matter". It's just a description of something mysterious causing effects that some kind

I think dark matter is a synonym for "undetectable-through-normal-means matter". It's just a description of something mysterious causing effects that some kind of matter would cause. It could be a mix of different kinds of undetectable matter, or some fundamental flaw in our understanding of matter and gravity, or something else. The arguments against aliens and mistakes are presumably that the effects appear consistently across the whole visible universe, and through many different measurements.