Detecting Dark Matter
Douglas Finkbeiner, Harvard-Smithsonian Center for Astrophysics

One of the most remarkable discoveries in all of astronomy and physics in the last century is the existence of "dark matter." Dark matter interacts with ordinary matter gravitationally, but other types of interactions are so tremendously weak they have never been detected, even by decades of ambitious experiments. Dark matter is not dark because it fails to reflect light (like a lump of coal) or is unilluminated for some reason (like an orphan planet coasting through interstellar space). Dark matter is a fundamentally different material than the protons, neutrons, and electrons that make up the visible Universe. Amazingly, there is 5 times as much dark matter in the Universe as ordinary matter. We are in the minority!

I will explain three reasons we think dark matter exists, and discuss why alternatives (such as modified laws of gravity) cannot be the explanation. I will then introduce the Weakly Interacting Massive Particle (WIMP), our best candidate for dark matter. Even though WIMPs interact very weakly with ordinary matter and light, there are still ways to detect them. On rare occasion, WIMPs could scatter off of atomic nuclei in very sensitive detectors deep underground.
Particle accelerators could produce WIMPs, or other new particles closely related to them. Finally astronomers observing high-energy gamma-rays and cosmic-rays may be able to detect the remains of certain kinds of WIMP interactions. All of these strategies are being vigorously pursued, and we hope to know much more about dark matter in coming years.

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