Learn About Dark Matter

Seventy years ago, astronomers noticed a discrepancy between what the laws of gravitation predict and the actual behavior of celestial matter. The discrepancy lies in the data points: not only do stars rotate around their galaxy's center more quickly than they ought to, but their velocities also do not fall off after the radius of the galaxy, but remain relatively constant. This is known as the galaxy rotation problem. There is simply not enough known matter to provide enough gravitational pull to hold galaxies together!

This missing matter also reveals itself in an effect called gravitational lensing. Massive objects, like a galaxy, between distant galaxies and earth gravitationally bend the light making the galaxies appear "lensed" when observed from earth. Studies have shown that the amount of known matter is not enough to account for these observations. A lensed galaxy is shown below.

Known matter, which is roughly synonymous with baryonic matter, refers to matter that we can see - matter that interacts with light. From the 2003 Wilkinson Microwave Anisotropy Probe (or WMAP), which mapped the Cosmic Microwave Background, we know that baryonic matter accounts for 4% of the matter-energy makeup of the universe; dark energy, 73%; and the largely mysterious dark matter, 23% - six times the amount of visible mass in the universe. This additional quantity of dark matter, if it exists, would supply the additional gravitational force needed to hold the universe together. WMAP results are shown below.

The CDMS experiment aims to detect one dark matter candidate, the WIMP: Weakly Interacting Massive Particle. These theoretical particles are thought to have a mass between 50 and 1000 GeV. They and are present at all places in the universe, and as cold dark matter, travel at speeds about 100 times slower than the speed of light. Because they are dark matter, the particles respond to neither the electromagnetic nor the strong nuclear force; they are weakly interacting and only react with baryonic matter through gravity and direct nuclear collisions. This means that these interactions are few and far between: a WIMP may pass through half a mile of solid rock without incident. This is why CDMS II is able to take place deep underground: the rock shielding has little to no effect on the WIMPs the experiment is trying to detect.

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