At present, the standard cosmological paradigm is a universe in which ordinary matter is a minor constituent, with of the mass being in some non-baryonic form. This is usually presumed to be some new fundamental particle (e.g., WIMPs or axions), which in the astronomical context is generically referred to as cold dark matter (CDM). ``Standard'' CDM began as a compelling and straightforward theory with few moving parts (e.g., Blumenthal et al. 1984BFPR). It has evolved into a model () with many fine tuned parameters (e.g., Ostriker & Steinhardt 1995OS). This might reflect our growing knowledge of real complexities, or it might be a sign of some fundamental problem.
As yet, we have no direct indication that CDM actually exists. Consequently, the assumption that it makes up the vast majority of mass in the universe remains just that: an assumption. The presumed existence of CDM is a well motivated inference based principally on two astrophysical observations. One is that the total mass density inferred dynamically greatly exceeds that allowed for normal baryonic matter by big bang nucleosynthesis (). The other is that the cosmic microwave background is very smooth. Structure can not grow gravitationally to the rich extent seen today unless there is a non-baryonic component which can already be significantly clumped at the time of recombination without leaving incriminatingly large fingerprints on the microwave background.
Nevertheless, CDM faces some severe problems, especially at smaller scales (e.g., Moore 1994Ben; Flores & Primack 1994FP; McGaugh & de Blok 1998aMBa; Moore et al. 1999MQGSL; Navarro & Steinmetz 2000NS; Sellwood 2000Jerry). Since the existence of CDM remains an assumption, it seems prudent to consider the case of a purely baryonic universe. In this context, it is not surprising that the second peak is constrained to have a small amplitude in the data reported by recent microwave background experiments (de Bernardis et al. 2000Boom; Hanany et al. 2000maxima). It is expected (McGaugh 1999mypred).