The Oort Limit

During galaxy formation non-baryonic dark matter is not expected to pack down into a thin disk. Hence the mass-to-light ratio ϒ d of the Galactic disk is a key number as the mass-to-light ratio of a purely baryonic system. If ϒ d proves to be large, it suggests that the large values of ϒ that are detected elsewhere in the Universe are due to dark baryonic matter rather than some exotic form of matter. Conversely, a small value ϒ d lends credence to the idea that most of the mass in the Universe is non-baryonic by suggesting that baryonic systems are well endowed with luminous stars.