Adapted from P. Coles, 1999, The Routledge Critical Dictionary of the New Cosmology, Routledge Inc., New York. Reprinted with the author's permission. To order this book click here: http://www.routledge-ny.com/books.cfm?isbn=0415923549
According to some models of cosmological structure formation, the Universe is dominated by an unseen component of dark matter which is in the form of weakly interacting massive particles (WIMPs). Although it is invisible by virtue of being unable to produce electromagnetic radiation, this material can in principle be detected by its gravitational effect on visible matter. But calculating the amount of dark matter in this way is a difficult business, particularly if the object in question has many different components, such as stars and gas as well as the putative WIMPs.
One kind of astronomical object that permits a detailed inventory to be made of its component matter is a massive cluster of galaxies such as the Coma Cluster (see large-scale structure). The Coma Cluster is a prominent concentration of many hundreds of galaxies. These galaxies are moving around in a hot plasma whose presence is detectable by X-ray astronomy methods. The luminous matter in the galaxies and the more tenuous plasma in the intracluster medium are both made of baryons, like all visible matter. As would be expected in a Universe dominated by WIMPs, baryonic cluster matter is only a small part of the total mass, total mass of the cluster can be estimated using dynamical arguments based on the virial theorem. This is used to infer the total mass of the cluster from the large peculiar motions of the component galaxies. It does not matter if the galaxies are not responsible for the mass in order for this to be done. All that is necessary is that they act like test particles, moving in response to the gravity generated by whatever mass is there.
When such a detailed audit of the mass of the Coma Cluster was
carried out, the conclusion was that the baryonic components
contributed about 25% of the total mass of the cluster, a result that
was dubbed the baryon catastrophe by scientists responsible for
analysing the data. So what is catastrophic about this result? The
answer relates to the theory of primordial nucleosyntbesis, one of the
main pillars upon which the Big Bang theory is constructed. The
predictions of calculations of the light element abundances produced
in the early stages of the primordial fireball agree with observations
only if the fractional contribution of baryons, 
b to the critical
density (see density parameter) is only 10% or so. According to some
models of structure formation, the total density parameter, , is
equal to 1 (so that we live in a flat universe), which means that 90%
of the mass of the Universe is in the form of WIMPs. What is more, in
these theories there seems to be no way of concentrating baryons
relative to the non-baryonic matter in an object the size of the
cluster. So the fraction of baryons in the Coma Cluster should be no
more than 10% or so. The observed value (25%) therefore appears to
rule out a Universe with =
1. While this conclusion may be
catastrophic for those die-hard adherents of a flat universe, many
others simply take the view that we must be living in an open universe
with = 0.2 or so. The fraction
of baryons in the Coma Cluster would
then be reconcilable with the 10% of the critical density it needs to
be in order to fit with nucleosynthesis calculations.
Similar studies have been carried out on other clusters which cast some doubt on the original interpretation of data from Coma. These studies show that the baryon fraction seems to vary significantly from cluster to cluster, which it should not do if it represents the global fraction of baryonic matter in the Universe at large.
FURTHER READING:
White, S.D.M. et al., `The baryon content of galaxy clusters: A challenge to cosmological orthodoxy', Nature, 1993, 366, 429.