Baryonic dark matter

The total amount of baryonic dark matter can be inferred from models of 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.

From the perspective of Big Bang nucleosynthesis, a larger amount of ordinary (baryonic) matter implies a denser early universe, more efficient conversion of matter to helium-4, and less unburned deuterium remaining. If all of the dark matter in the universe were baryonic, then there would be much less deuterium in the universe than is observed. This could be resolved if more deuterium were somehow generated, but large efforts in the 1970s failed to identify plausible mechanisms for this to occur. For instance, MACHOs, which include, for example, brown dwarfs (bodies of hydrogen and helium with masses less than 0.08 M_☉ or 1.6×10²⁹ kg), never begin nuclear fusion of hydrogen, but they do burn deuterium. Other possibilities that were examined include "Jupiters", which are similar to brown dwarfs but have masses ${\displaystyle \sim }$ 0.001 M_☉ (2×10²⁷ kg) and do not burn anything, and white dwarfs.^{[1][2][clarification needed]}

• Particle chauvinism