Most of the matter in the universe is dark. Without dark matter, galaxies and stars would not have formed and life would not exist. It holds the universe together. What is it?
Although the existence of dark matter was suggested in the 1930s, only in the last 10 to 15 years have scientists made substantial progress in understanding its properties, mostly by establishing what it is not. Recent observations of the effect of dark matter on the structure of the universe have shown that it is unlike any form of matter that we have discovered or measured in the laboratory. At the same time, new theories have emerged that may tell us what dark matter actually is. The theory of supersymmetry predicts new families of particles interacting very weakly with ordinary matter. The lightest supersymmetric particle could well be the elusive dark matter particle. We need to study dark matter directly by detecting relic dark matter particles in an underground detector and by creating dark matter particles at accelerators, where we can measure their properties and understand how they fit into the cosmic picture.
Most of the matter in the universe is dark. Early evidence for dark matter came from the rotation curves of galaxies, which showed that galaxies contain more mass than is contained in the stars. More recently, direct evidence for dark matter has come from the discovery and characterization of gravitational lenses, regions of space where mass bends light. These astronomical constraints do not directly distinguish between nonbaryonic models for dark matter (WIMPs) and other possible ideas involving more massive objects (MACHOs) such as Jupiter-sized planets and mini-black holes. However experiments in the 1990s established that MACHOs do not make an appreciable contribution to the dark matter content of our galaxy.
The tightest constraints on the amount of dark matter in the universe come from cosmological measurements. The frequency and amplitude dependence of the fluctuations in the cosmic microwave background (CMB) measured by WMAP are sensitive to both the total matter density and the baryon density. The baryon density is also constrained by the nucleosynthesis models of the early universe. All of these methods suggest that normal baryonic matter can only account for a small fraction, about five percent, of the total matter density.
Scientists are measuring the distribution of dark matter in the universe in a variety of ways: (a) by studying the large-scale distribution of galaxies, as with the Sloan Digital Sky Survey (SDSS); (b) by constraining the dark matter mass power spectrum through weak lensing studies, as by a proposed Large Synoptic Survey Telescope (LSST) and the Joint Dark Energy Mission (JDEM); and (c) by cataloging massive clusters of galaxies as a function of redshift, using the Sunyaev-Zeldovitch effect, by the South Polar Telescope and the Atacame Cosmology Telescope.
What is dark matter? Particle physics models suggest that dark matter is either axions (hypothetical new particles associated with Quantum Chromodynamics ), or WIMPs (hypothetical new particles with weak interactions and TeV-scale masses, natural by-products of theories of supersymmetry or extra dimensions). If dark matter particles are relics from the near-total annihilation in the early universe, simple dimensional analysis suggests that the particles originate from physics at the TeV scale. The particle nature of dark matter can be verified by finding the rare events they would produce in a sensitive underground dark matter detector such as Cryogenic Dark Matter Search . Such experiments may see products of dark matter particles in our galaxy. Annihilation of TeV-scale dark matter particles might be detected as line radiation in high-energy gamma ray telescopes such as GLAST and VERITAS, or possibly in astrophysical neutrino detectors such as IceCube. Antiparticles produced in these annihilations might also be detectable by AMS. If dark matter particles are much more massive, they might produce signals in the ultra-high-energy cosmic rays.
However, to understand the true nature of dark matter particles, particle physics experiments at accelerators such as the LHC must produce them and study their quantum properties. Physicists need to discover how they fit into a coherent picture of the universe. Suppose experimenters detect WIMPs streaming through an underground detector. What are they? Are they the lightest supersymmetric particle? The lightest particle moving in extra dimensions? Or are they something else?
Searches for candidate dark matter particles are underway at present-day colliders. If these particles have masses at the TeV scale, they will surely be discovered at the LHC. Verifying that these new particles are indeed related to dark matter will require a linear collider to characterize their properties. A linear machine would be able to measure the mass, spin and parity of dark matter particles with precision. These results will permit calculation of the present-day cosmic abundance of dark matter and comparison to cosmological observations. If the values agree, it will be a great triumph for both particle physics and cosmology and will extend the understanding of the evolution of the universe back to 1010 seconds after the big bang.