Measurements of the mass-to-light ratios in clusters suggest that , the ratio of the total density of the universe to the critical density, exceeds 0.2 . This determination of is consistent with measurements based upon the large-scale velocity fields and the dynamics of the large-scale structure . Values of less than 0.2 are very difficult to reconcile with the 500 km/s random velocities seen in large scale structure surveys and even harder to reconcile with large-scale streaming motions.
The observed (presumed cosmological) abundances of deuterium, helium and lithium are only consistent with standard big bang nucleosynthesis if the baryon density is much less than . The best fit value for , which is nearly an order of magnitude below the dynamical values . For example, if km/s/Mpc, implies that Y, the Helium/Hydrogen abundance ratio, is and , the Deuterium/Hydrogen abundance ratio, is  while if km/s/Mpc, implies and . There are many extragalactic HII regions with and best estimates imply . These observations appear to require either a significant modification of our ideas about big bang nucleosynthesis or the existence of copious amounts of non-baryonic dark matter. (See, however, Goldwirth & Sasselov  for a dissenting view).
All of the proposed modifications of BBN appear to violate known observational constraints. For example, Gnedin & Ostriker  proposed that an early gamma-ray background photodissociated some of the primordial Helium. This model predicts a spectral distortion of and a fully ionized universe. y describes the deviation of the observed spectrum from the thermal spectrum and is a measure of the energy injection in the early universe. COBE  found that the observed spectrum was consistent (within the experimental errors) with a thermal spectrum and constrained .
Inhomogeneous nucleosynthesis models have been studied extensively in the past few years. However, Thomas et al.  found that even models with large inhomogeneities imply for . Thus, they are also not consistent with .
While the theory of big bang nucleosynthesis is well developed, there is still uncertainty in converting the observed line ratios to abundances. Most of the abundances for external systems assume a spherical clouds with constant rates of ionization. It would be interesting to study a nearby system such as the Orion nebula and estimate the error associated with this approximation in the analysis. Goldwirth & Sasselov  have made an important first step in studying the sensitivity of these element abundances to model uncertainties. There is a need for more work.
While none of these three arguments is incontrovertible, they all do suggest that most of the universe is in non-baryonic matter. The rest of this paper will review the most popular proposed candidates for non-baryonic dark matter and consider various schemes for detecting its presence.