<Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (March 2017) (Learn how and when to remove this template message) </Td> </Tr> <P> Deuterium is in some ways the opposite of helium - 4, in that while helium - 4 is very stable and difficult to destroy, deuterium is only marginally stable and easy to destroy . The temperatures, time, and densities were sufficient to combine a substantial fraction of the deuterium nuclei to form helium - 4 but insufficient to carry the process further using helium - 4 in the next fusion step . BBN did not convert all of the deuterium in the universe to helium - 4 due to the expansion that cooled the universe and reduced the density, and so cut that conversion short before it could proceed any further . One consequence of this is that, unlike helium - 4, the amount of deuterium is very sensitive to initial conditions . The denser the initial universe was, the more deuterium would be converted to helium - 4 before time ran out, and the less deuterium would remain . </P> <P> There are no known post-Big Bang processes which can produce significant amounts of deuterium . Hence observations about deuterium abundance suggest that the universe is not infinitely old, which is in accordance with the Big Bang theory . </P> <P> During the 1970s, there were major efforts to find processes that could produce deuterium, but those revealed ways of producing isotopes other than deuterium . The problem was that while the concentration of deuterium in the universe is consistent with the Big Bang model as a whole, it is too high to be consistent with a model that presumes that most of the universe is composed of protons and neutrons . If one assumes that all of the universe consists of protons and neutrons, the density of the universe is such that much of the currently observed deuterium would have been burned into helium - 4 . The standard explanation now used for the abundance of deuterium is that the universe does not consist mostly of baryons, but that non-baryonic matter (also known as dark matter) makes up most of the mass of the universe . This explanation is also consistent with calculations that show that a universe made mostly of protons and neutrons would be far more clumpy than is observed . </P>

Sequence the following products of big bang nucleosynthesis from lowest to highest by mass