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The search for dark matter

Although a few astronomers think that Newton’s laws need to be modified, most suspect that we just haven’t found the majority of the universe’s matter.
RELATED TOPICS: DARK MATTER
Bullet cluster of galaxies
Two massive galaxy clusters collided to form what’s known as the Bullet cluster of galaxies. Ordinary matter — the hot gas, shown in pink — collided, lost energy, and slowed. The clusters’ dark matter (shown in blue) interacted little and passed through the ordinary matter.
NASA/ESA/CXC/M. Bradac (UCSB)/S. Allen (Stanford Univ.)
When you don’t understand something, sometimes the most human response is to ignore it. For decades, many scientists proved all too human. Astronomers looked to the distant universe and saw clusters containing thousands of galaxies. Each of these systems seems to be bound gravitationally, yet the visible material in the cluster members accounts for only 10 percent or so of the amount required to do the job. Closer to home, the outer regions of most spiral galaxies revolve faster than Newton’s laws predict based on the amount of material astronomers can see.

As more and better observations accumulated, scientists realized that they couldn’t ignore the problem any longer. The search to find so-called dark matter — invisible material that nevertheless exerts a gravitational pull — began in earnest. At first, researchers expected the dark matter to be normal matter that just didn’t emit much light, such as brown dwarfs (failed stars that don’t have enough mass to fuse hydrogen), rogue planets, and isolated black holes. But today, astronomers suspect that dark matter isn’t like the material that makes up stars, planets, and humans. Instead, the best hypothesis is that weakly interacting massive particles, aka “WIMPs,” make up this missing material. Astronomy covered the search for this enigmatic stuff in “What do we really know about dark matter” in the November 2009 issue.
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