# How do we know Dark Matter is non baryonic?

It seems widely stated, but not thoroughly explained, that Dark Matter is not normal matter as we understand it. Wikipedia states "Consistency with other observations indicates that the vast majority of dark matter in the universe cannot be baryons, and is thus not formed out of atoms."

How can we presume to know this? Our best evidence for such dark matter is the rotational speeds of galaxies. It sounds like we can measure/approximate the gas density and stellar masses somehow, yet I don't understand how we can account for things like planets, asteroids, black holes without accretion disks, and other things that have mass but don't glow. How is it we dismiss these explanations for it, and jump right to WIMPs and other exotic explanations?

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 You want to search on "Micro-lensing" and "MACHOs" in conjunction. The density of compact, cold baryonic object in middle masses is well measured for the Milky Way. It is far below that needed to account for the rotation curve. – dmckee♦ Apr 5 '12 at 19:45 Possible duplicate: physics.stackexchange.com/q/1008/2451 – Qmechanic♦ Feb 2 at 11:57

Gravitational-lensing searches show that the "dark-matter" constituents must be composed of objects less than about $10^{-7} \textrm{ M}_\odot \sim 0.03 \textrm{ M}_\oplus$, i.e. it must be asteroid size or smaller. Asteroid size can't really form stably (in such large amounts), and would be rapidly accreted by larger mass objects --> dark-matter constituents must be small.