Non-Baryonic Dark Matter

Non-Baryonic Dark Matter:
An alternative idea is to consider forms of dark matter not composed of quarks or leptons, rather
made from some exotic material. If the neutrino has mass, then it would make a good dark matter
candidate since it interacts weakly with matter and, therefore, is very hard to detect. However,
neutrinos formed in the early Universe would also have mass, and that mass would have a
predictable effect on the cluster of galaxies, which is not seen.
Another suggestion is that some new particle exists similar to the neutrino, but more massive and,
therefore, more rare. This Weakly Interacting Massive Particle (WIMP) would escape detection in
our modern particle accelerators, but no other evidence of its existence has been found.

The more bizarre proposed solutions to the dark matter problem require the use of little understood
relics or defects from the early Universe. One school of thought believes that topological defects
may have appears during the phase transition at the end of the GUT era. These defects would have
had a string-like form and, thus, are called cosmic strings. Cosmic strings would contain the
trapped remnants of the earlier dense phase of the Universe. Being high density, they would also be
high in mass but are only detectable by their gravitational radiation.
Lastly, the dark matter problem may be an illusion. Rather than missing matter, gravity may
operate differently on scales the size of galaxies. This would cause us to overestimate the amount
of mass, when it is the weaker gravity to blame. This is no evidence of modified gravity in our
laboratory experiments to date.