So I just finished watching "dark matter is not a theory". An understanding I gleaned from it is that dark matter is observed from the discrepancy between the amount (or perhaps energy? in terms of spectrum) of light a galaxy is emitting, and its rotational speed. These are both indications of mass, and they should match up, but the fact that they don't indicates that there is an extra mass there, but one which does not emit light.

Long story short, I have a friend who believes that dark matter is just "regular" dust that is blocking light emitted from the galaxies with observed dark matter.

What are the arguments against dark matter just being "dust", or non-star matter? I know there are a number of "exotic" theories, such as unknown/unpredicted particles types (WIMPs, MaCHOs) and such-- why is a more prosaic solution not preferred over these?