[mikeash]

Exoplanets and asteroids don't explain the discrepancies. There aren't enough of them. Our galaxy appears to have about 20x more dark matter than regular matter, yet our solar system has about 99.9% of its mass concentrated in its star. There's no indication other star systems with planets are radically different in that respect. If there was enough of this stuff to make up the discrepancy, there would be enough of it to see it.

There's also apparently decent evidence, based on things like irregularities in the cosmic microwave background, that most dark matter isn't any form of baryonic matter.