# Baryonic 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 with plausible mechanisms for this to occur. For instance, MACHOs, which include, for example, brown dwarfs (balls of hydrogen and helium with masses $< 0.08M_\odot$), 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 $\sim 0.001M_\odot$ and do not burn anything, and white dwarfs.[1][2][clarification needed]