Baryonic dark matter
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Baryonic dark matter is the dark matter (matter that doesn't emit light) composed of baryons, i.e. protons and neutrons. Candidates for baryonic dark matter are non-luminous gas, MACHOs, and brown dwarfs.
The total amount of dark matter can be calculated from big bang nucleosynthesis, and observations of the cosmic microwave background. Both indicate that the amount of baryonic dark matter is much smaller than the total amount of dark matter.
In the case of big bang nucleosynthesis, the problem is that large amounts of ordinary matter means a denser early universe, more efficient conversion of matter to helium-4 and less unburned deuterium that can remain. If one assumes that all of the dark matter in the universe consists of baryons, then there is far too much deuterium in the universe. This could be resolved if there were some means of generating deuterium, but large efforts in the 1970s failed to come up plausible mechanisms for this to occur.