Unambiguous direct detection of cosmological dark matter particles is, perhaps, the single most pressing particle astrophysics measurement sought by cosmologists and theoretical particle physicists alike. Dark matter is well known by its gravitational effects observed throughout the universe, despite being entirely invisible to our telescopes measuring the electromagnetic spectrum from radio frequencies, through the visible spectrum, and up to gamma rays. The gravitational effects appear in deviations from uniform intensity of the cosmic microwave background radiation, galactic clustering, gravitational lensing, and the rotation of galaxies. The leading candidate for these gravitational effects is the presence of a weakly interacting massive particle (WIMP), the gravitational influence of which is ubiquitous but otherwise does not participate in the familiar interactions of the Standard Model of particle physics.
The public and scientific interest in dark matter stems from the glaring realization that scientists and researchers only know and understand ~5% of the material that composes the universe. The figure above divides the universe into categories of mass composition. From the last century of physical research, science can claim familiarity with only the heavy elements, neutrinos, stars, and free hydrogen and helium produced by the Big Bang. The remaining ~95% is essentially unknown. Dark matter, at 23%, is better understood—more fairly, better constrained—than the mysterious dark energy component.
The search for WIMP dark matter has more than a twenty-five year of history, dovetailing on cosmologists' efforts to account for the evolution of the mass distribution of the universe. Where these observations have led to proposals for a particle physics solution, a confrontation with the Standard Model of particle physics is inevitable. No known particle has properties that account for the astronomical observations, yet a clear prediction emerges that WIMP dark matter should interact with the matter of our everyday experience via elastic scattering. Simply put, a heavy dark matter particle should bounce off nuclei nearly as if they were two billiard balls on the physicist's pedagogical pool table.
As gravitational evidence has solidified and the direct measurement of cosmological dark matter remains elusive, calls for an experimental resolution to this mystery have grown. Written for the National Academies, the 2006 report, Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics1, devotes one of four "Key Question in Particle Physics" to determining the nature of dark matter. A slightly older report, Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century2 significantly poses its first question as: "What is Dark Matter?"
1 U.S. National Research Council, Committee on Elementary Particle Physics in the 21st Century. 2006. Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics. National Academies Press, Washington, D.C.
2 U.S. National Research Council, Committee on the Physics of the Universe. 2003. Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century. National Academies Press, Washington, D.C.