Quasar absorption lines now permit a direct probe of deuterium abundances in primordial material , with the best current estimate ( D / H ) = 1.9 \pm 0.4 \times 10 ^ { -4 } .
If this is the universal primordial abundance ( D / H ) _ { p } , Standard Big Bang Nucleosynthesis yields an estimate of the mean cosmic density of baryons , \eta _ { 10 } = 1.7 \pm 0.2 or \Omega _ { b } h ^ { 2 } = 6.2 \pm 0.8 \times 10 ^ { -3 } , leading to SBBN predictions in excellent agreement with estimates of primordial abundances of helium-4 and lithium-7 .
Lower values of ( D / H ) _ { p } derived from Galactic chemical evolution models may instead be a sign of destruction of deuterium and helium-3 in stars .
The inferred baryon density is compared with known baryons in stars and neutral gas ; about two thirds of the baryons are in some still-unobserved form such as ionized gas or compact objects .
Galaxy dynamical mass estimates reveal the need for primarily nonbaryonic dark matter in galaxy halos .
Galaxy cluster dynamics imply that the total density of this dark matter , while twenty or more times the baryon density , is still well below the critical value , unless both baryons and galaxies are concentrated in galaxy clusters relative to the dark matter .