What might the dark matter be?
Answers
Answer:
The realization that the majority ofthe matter in the Universe might be non-baryonic is the ultimate Copernican viewpoint; not only are we in no special place in the Universe, but we aren't even made out of the same stuff as dominates the matter density of the Universe. The prediction of non-baryonic dark matter is one of the boldest and most striking in all of cosmology, and if ultimately verified, for example by direct detection of dark matter particles, will be amongst cosmology's most notable successes.
Fundamental particles
• Things we know exist: The particle which we know exists and yet whose properties are uncertain enough to allow it to be the dark matter is the neutrino. In the Standard Model of particle interactions the neutrino is a massless particle, and is present in the Universe in great abundance, being about as numerous as photons oflight. Ifthe Standard Model is extended to permit the neutrinos to have a small mass (a few tens of electron-volts), this would not affect their number density but they would have enough density to imply a closed Universe! The required density is comparable to, or perhaps slightly higher than, current experimental limits on the electron neutrino. but there are also the neutrinos associated with the muon and tau particles.