In the following section it will be seen that we do not yet understand what determines the gravitational potentials within the Universe on scales from galaxies upwards. One suggestion is that the Universe is full of material that does not emit electromagnetic radiation and which is thus hard to ‘see’. There are a tremendous number of ideas on offer to resolve the dark matter dilemma. These ideas cover modifications to the laws of gravity, cold conventional matter, new particles, neutrinos with non-zero mass, and exotic objects, to name just a few. These will be discussed in more detail later. At the moment, there is no absolutely clear solution, although a number of the possibilities do lend themselves to experimental “searches’,” which are being carried out around the world. Some of the experiments are designed to look for direct signatures, i.e. the physical interaction of a dark matter particle in a detector, while others are looking for indirect evidence, such as the neutrino or -ray flux due to dark matter particle annihilations. In addition, other types of experiments are looking for circumstantial evidence, such as the measurement of a non-zero neutrino mass in the right range, or confirmation of supersymmetry (SUSY). A number of the direct and indirect searches will be digging deeply into theoretically very interesting regions of parameter space over the next five years. Indeed there is, at this time, one experimental result from a direct search that has a signal consistent with the most optimistic SUSY predictions concerning neutralinos – which just might be proven correct. While it is true to say that most activity is currently targeted at what might be called the standard cosmological model, this model is not without its difficulties as a theory, and experimental evidence from dedicated dark matter searches could be crucial in finally clarifying the situation. Experimental searches probably are the only way to find out unambiguously what the Milky Way is made of.
The evidence is reviewed in Section 2, including Subsection 2.2 on the standard cosmological model. The candidates are presented in Section 3. This leads to Section 4 on direct and indirect detection requirements and techniques for non-baryonic dark matter. Most experiments are being carried out in the context of theoretical expectations for the neutralino, which most regard as the best motivated of the particle candidates. Section 5 looks in detail at the requirements for detection of the neutralino. A review of the current status of projects world-wide will be given in Section 6. Finally, in Section 7 there is a discussion of the next logical steps for experimental searches for dark matter.
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