There's one other thing that goes against the idea of "dark dust". if it was "dark dust" that was just a sparse cloud of standard elements, we would be able to

There's one other thing that goes against the idea of "dark dust". if it was "dark dust" that was just a sparse cloud of standard elements, we would be able to detect the makeup of the cloud by pointing a Spectrometer at some of the darker (but seemingly "denser") parts of the cloud to detect what it's made of.