It has been well established for decades that the majority of the matter in the Universe is dark. This dark matter is the gravitational glue that holds together galaxies and galaxy clusters and is the key ingredient in the successful cold dark matter (CDM) paradigm of structure formation. Observations of the CMB and of the light element abundances from big bang nucleosynthesis indicate that most of the dark matter is non-baryonic; a new form of matter outside of the Standard Model of particle physics. Many possibilities for dark matter have been proposed. The most popular hypothesis of the last several decades is that the dark matter is a cold relic of the big bang, a new Weakly Interacting Massive Particle, or WIMP. A major goal of the KICP’s dark matter efforts is to confirm or refute the hypothesis that dark matter consists of WIMPs, and if discovered, to understand how they fit into extensions of the Standard Model of particle physics. Several attractive extensions of the standard model, most notably supersymmetry (SUSY) predict the existence of WIMPs with properties remarkably in accordance with those needed to obtain the current matter density.
A variety of observations and experiments should either confirm, or come close to excluding, the WIMP hypothesis in the near future. After many years of effort, experiments based on direct detection of relic WIMPs via scattering with nuclei in sensitive, low-background detectors such as XENON, PICO, and DAMIC are finally reaching the sensitivity needed to probe the expected region of WIMP parameter space in SUSY and other models. If the WIMP was produced in the primordial soup of the big bang, it should also be produced in collisions of high-energy particles at facilities such as the LHC. WIMPs may be identified by their missing energy signatures at colliders. Finally, the products of WIMP annihilations in the cosmos can be observed by detectors of high- or low-energy photons, positrons, antiprotons, or high-energy neutrinos. These indirect detection techniques are now reaching the sensitivities needed to probe the signals expected from WIMP annihilations.
To date, the various approaches mentioned have provided only limits and exclusions. This has led to numerous WIMP alternatives becoming increasingly interesting as dark matter possibilities. Axions, dark sector particles, and numerous others are seeing increasing study and efforts to develop strategies for their detection.