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Q&A: Dark Matter

Q:
Can WIMPs solve the dark matter problem?

A:
Yes, WIMPs could solve the dark matter mystery, but not just any old WIMP will do. The only type of WIMP that has been observed (maybe) is one type of neutrino with mass. However, the mass of this neutrino is too small to have any effect on the dark matter problem, so WIMPs remain a theoretical concept. The WIMP with the most-favored status these days is a cold WIMP, meaning that it is moving slowly, so it clumps easily into galaxy sized masses which then gather together over billions of years to form galaxy clusters. The problem is, they don't seem to form galaxy clusters quickly enough, so maybe a few hot (fast moving) WIMPs are needed to help form big structures. Another problem with WIMPs is that the theoretical predictions all insisted that the Big Bang would produce enough WIMPs to give the universe the critical density (critical density = density that is the dividing line between a universe that expands forever, and one that will ultimately collapse). This prediction is apparently not true --observations indicate that the density is 3 or 4 below the critical value. The most recent models for the universe have a mixture of ordinary matter, some WIMPs to solve the dark matter problem, and a cosmic "dark energy" that causes the expansion of the universe to speed up. If you find this confusing, then you are in good company. Most astrophysicists don't know what to make of it either, but they are optimistic that a host of new telescopes including Chandra, will in the coming decade pin down how much dark matter and dark energy there is in our universe. Whether they will then understand what it is, is another question. Refer to the dark matter mystery section for more info at http://chandra.harvard.edu/xray_astro/dark_matter.html

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