Discrepancies between the apparent baryon content of galaxies and galaxy clusters and their dynamically inferred masses are generally attributed to dark matter (DM). A still larger mass discrepancy exists out to the horizon scale, since Big Bang nucleosynthesis requires a low baryonic mass fraction in the universe (Tytler et al. 2000), which nevertheless appears to have flat geometry (de Bernardis et al. 2000; Hanany et al. 2000). The missing energy density required by conventional cosmology is now supposed to reside not only in non-baryonic particles, but also partly as vacuum energy.
The currently popular CDM model for the evolution of structure in the universe is quite successful on large scales (e.g. Bahcall et al. 1999), but its predictions on galaxy scales disagree with observed properties of galaxies. A number of lines of evidence suggest that the problem is more fundamental than simply our inadequate understanding of the process of galaxy formation.