Evidence: Much evidence now supports the existence of dark matter, the earliest of which was observing the rotation and speed of celestial bodies; various stars, galaxies and other bodies were observed to move much faster than expected. These bodies were found to move with an approximately constant speed towards the largest measurable radius of rotation, instead of following the expected 1/√r proportionality. Other effects such as gravitational lensing and the imprint from cosmic microwave background also allude to the existence of a mass that only interacts gravitationally. Overall, dark matter is thought to make up as much as 30% of matter in the universe.
Candidates: A favorite hypothesis for the nature of Dark Matter is that it is composed of particles that are known to not interact through the electromagnetic force, but which could be subject to a new -weak- force. Principal candidates include Weakly Interacting Massive particles (WIMPs), Axions, and sterile neutrinos; a large number of other candidates has also been put forward.
WIMPs (χ) are stable particles with masses of about few GeV to few TeV, which were in thermal and chemical equilibrium early in the history of the Universe and which would be presently clumped within large scale structures, ie galaxies and clusters of galaxies. A favorite particle physics WIMP could be the lightest neutralino, the stable particle of SuperSymmetry theories. Other non-super symmetric WIMPs, in particular originating fom Dark Sector models, are also hypothesized.
Axions were proposed by Peccei-Quinn to solve the issue of charge parity violations in strong interactions as predicted by quantum chromodynamics. Axion properties include that they must be stable, have a very low mass and cross section, and couple to photons. If the mass of axions fall within a specific range, they could be the main component in dark matter.
The Kaluza-Klein theory is a precursor to string theory, in which (in the simplest example) a fifth spacial dimension exists and forms a circle, producing cylindrical coordinates. This model predicts the existence of axions, theoretical particles proposed to explain several cosmological phenomena, including long-standing anomalies in X-ray observations from cosmological sources.
Some models predict two photon decay, to which a detector like a large volume SPC can be particularly sensitive to.