Seeing the Unseeable — Part I
So far everything astronomers know about dark matter comes from its gravitational effects on visible matter. But they need to detect it directly if they are to find out what it is. Motivated by the promise of discovering what a quarter of the universe is, thousands of researchers are looking. Most of their efforts have focused on WIMPs, and the three common search strategies are to look for the particles' annihilation, scattering and production.

When two WIMPs meet, they obliterate each other and leave behind a clutch of other particles such as electrons, antielectrons (known as positrons) and neutrinos. Such annihilation cannot be very common, or else no WIMPs would be left by now. Fortunately, current experiments are sensitive enough to notice if even a tiny fraction of WIMPs are being annihilated.
Detectors on high-altitude balloons and satellites have sought electrons and positrons. In the coming year the space shuttle is scheduled to transport the Alpha Magnetic Spectrometer to the International Space Station, where it will sit docked, looking for positrons. Other observatories such as the Super-Kamiokande experiment in Japan and IceCube in Antarctica are watching for neutrinos.
Welcome to the world of hidden particles
A stunning flight through the Universe
The Dark Side
The train of thought to dark matter began with the discovery of radioactive beta decay in the early 1900s. Italian theorist Enrico Fermi sought to explain the phenomenon by postulating a new force of nature and new force-carrying particles that caused atomic nuclei to decay. This new force was similar to electromagnetism and the new particles to photons—but with a key twist. Unlike photons, which are massless and therefore highly mobile, Fermi argued that the new particles had to be heavy. Their mass would limit their range and account for why the force causes nuclei to fall apart but otherwise goes unnoticed. The new force is now known as the weak nuclear force and the hypothesized force-carrying particles are the W and Z particles, which were discovered in the 1980's. They are not dark matter themselves, but their properties hint at dark matter.

One goal of the Large Hadron Collider is to look for those particles, which should have masses comparable to those of the W and Z. Indeed, physicists think dozens of types of particles may be waiting to be discovered—one for each of the known particles, paired off in an arrangement known as supersymmetry.

These hypothetical particles include some collectively known as weakly interacting massive particles, or WIMPs. The name arises because the particles interact only by means of the weak nuclear force. Being immune to the electric and magnetic forces that dominate the everyday world, they are totally invisible and have scarcely any direct effect on normal particles. Therefore, they make the perfect candidate for cosmic dark matter.
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Through the force of gravity, dark matter sculpts the universe into a web of galaxies. Theorists now suspect that it may exert other forces as well. This image from the Millennium Simulation project in 2005 depicts a region roughly 1.6 billion light-years across.