The mystery of dark matter, that most of the matter in the Universe appears only through its gravitational interactions, has been open since 1933. Particle physics provides a well-motivated hypothetical solution, in the form of Weakly Interacting, Massive, Particles: WIMPs. Experiments to detect directly the interactions of WIMPs have been underway for over twenty years, yet the WIMPs are still eluding us.
The challenge is twofold. First, the WIMPs only deposit a very small amount of energy in a detector. Second, the interaction rate is very low, less than one event per kilogram of detector per month compared to, for instance, normal levels of radioactivity which amount to several thousand events per second in a human body for instance. WIMPs detectors requires sophisticated technology with draconian shielding against background radiation.
Fast Fact: Dark matter comprises 23 per cent of the universe (while dark energy consists of 72 per cent). This matter, different from atoms, does not emit or absorb light. It has only been detected indirectly by its gravity.
Currently, the most sensitive of these direct detection experiments is called "CDMS", located deep in a mine in Minnesota. CDMS has recently observed two events that cannot be ruled out as WIMPs, but may also be some form of residual background. Many other experiments around the world have been planned to test this observation with a variety of independent techniques. Several of these experiments will be located at SNOLAB, an ultra-low background laboratory in a deeper mine in Sudbury, Ontario.
Fast Fact: SNOLAB is situated 2km underneath the surface and will have 5,000 metres squared of clean space underground for experiments and the supporting infrastructure.
Another, indirect, approach to finding dark matter is to look for traces left when WIMPs collide and annihilate, in the Milky Way or other galaxies. The PAMELA satellite, for example, has reported an excess of high-energy anti-matter positrons. While this excess could indicate the presence of dark matter, it could also have other astrophysical origins, such as pulsars, or supernovae. Overall, the clues to the nature of dark matter are multiplying, but there is no conclusive evidence yet.
This answer was written by Philippe Di Stefano, Associate Professor at Queen's University, a lead researcher in the area of dark matter detection.