Non-gravitational detection of dark matter

Anyone who thinks objectively about the concordance model of the Universe must surely be concerned that 80% of the matter content of the Universe has never been detected independently of its global gravitational effects in large, and generally distant, astronomical systems. It is a bit as though Neptune had never been discovered after having been postulated to account for the anomalous motion of Uranus. Not only has the non-baryonic particle matter never been seen (it is of course, dark), but we have no definite idea of the identity of these putative cold dark matter particles. As discussed in Chapter 6, there are no known “standard-model” particles that fit the bill – they must be electrically neutral, stable and slow moving (cold). But reasonable extensions of the standard model of particle physics do provide a number of candidates, and, of these, perhaps the most well-motivated is the LSP, the “lightest superpartner” that should exist in the context of the theory of supersymmetry.

To summarize the discussion in Section 6.4, the basis of supersymmetry is that every known particle has a partner that differs by a half-integral spin; for example, the partner of the spin 1 photon is the spin 1/2 photino. This proposed symmetry between integral spin particles, bosons, and half-integral spin particles, fermions, rather successfully confronts a number of theoretical problems in physics, although, so far, there is no direct experimental verification of supersymmetry. But the theory does, in effect, double the number of possible particles.