A 23% of all there is in the Universe is what we call Dark Matter. Dark because unlike normal matter (4% of all there is) it does not interact with light. So we cannot see it with our telescopes, although we can clearly see its imprint on the distribution and movement of the visible stars and galaxies through gravitational forces among them. But while in this way we can even map its distribution throughout the accessible history of our universe, we have no clear hints yet as to its true nature.
One possibility is that Dark Matter takes part in the weak interaction of particle physics. As this particular possibility is very attractive also from the point of view of particle theory, it has given rise to a variety of dedicated experiments around the world. WIMP has become the shorthand for this type of Dark Matter: Weakly Interacting Massive Particle.
Kavli IPMU is involved in this effort through the XMASS experiment at its Kamioka branch.In the XMASS detector one metric ton of xenon is kept as a liquid at minus 100 degree Celsius. The dense liquid provides both a large target mass at its center as well as shielding against radioactive backgrounds. Fast neutrons from the surrounding rock are moderated and remaining muons tagged in an 800 ton active water shield in which the detector is immersed.
The first data was taken in fall 2011. After several improvements and further tuning of the detector system, the detector would become the world's most sensitive detector for the direct detection of dark matter particles in our galaxy.