Re: Why are WIMPS and Nutrinos thought to be the majority of Dark Matter?

Dark matter and its nature have a long and complicated history. Some of the best evidence of the existence of dark matter is found in the rotation curves of galaxies and the dynamics of clusters of galaxies. However, this dark matter could be baryonic or non-baryonic. To resolve this, scientists have performed detailed calculations of nucleosynthesis in the early universe. These calculations combined with observations of the abundance of deuterium relative to hydrogen indicate that the amount of mass in baryons in the universe is about 3 percent of the critical density (the desnity beyond which th universe would eventually recollapse into a Big Crunch). Other observations of galaxies and galaxy clusters indicate that the total amount of matter in the universe is closer to 10-30 percent of the critical value.

Another piece of evidence pointing towards non-baryonic dark matter is the hot gas surrounding many clusters of galaxies. This gas contains a few times more mass than the stars in the galaxies in the cluster, but the total amount of baryonic matter (stars and hot gas) is only 10-30 percent of the total amount of matter. This rules out diffuse gas as a possible reservoir of baryonic dark matter. Could there be baryonic dark matter? Sure, and a large collaboration has been searching for it via gravitational microlensing.

Finally, an Italian group has announced tentative evidence of the direct detection of non-baryonic dark matter. An American group performing a similar study disputes their claim, and it's fair to say that the jury is still out on whether the claim is valid or not. Hopefully these teams along with the GLAST satellite will decipher the nature of dark matter with sufficient evidence that debates will be forgotten.