Second fiddle.
New dark matter findings show that our neighbor galaxy, Andromeda (shown here), is smaller than our own Milky Way.


Shining Light on Dark Matter

Researchers say they have for the first time measured physical properties of dark matter, the invisible stuff that makes up much of the universe. A team led by Gerry Gilmore of the Institute of Astronomy at Cambridge University detected a core of dark matter of uniform size and temperature inside dwarf galaxies. The finding runs counter to current dark matter theories, in part because the temperature measured was warmer than popular theories predict.

Astronomers have known for decades that the visible stars in a galaxy don't have enough gravity to hold it together. Large amounts of dark matter must make up the balance. But scientists have been stumped in their efforts to locate or describe it. The most popular theory suggests that dark matter consists of massive exotic particles that do not interact with normal matter except through gravity. It also holds that the particles are slow and cool. While this model fits most galaxies, it also predicts many more small galaxies than are known.

For the past 3 years, Gilmore and his team have been using giant telescopes to map the positions and velocities of thousands of stars in 10 minigalaxies around the Milky Way. Gilmore hadn't intended to make a big announcement, but on 3 February he appeared with others at a press conference in London to publicize the work of the European Southern Observatory (ESO), one of whose telescopes the team is using. At the press conference, Gilmore said the team had found the same volume of dark matter in each galaxy. The dark matter was about 1000 light-years across and had an even density equivalent to four hydrogen atoms per cubic centimeter.

The new results suggest that dark matter at the center of small galaxies is more spread out and warmer than was thought. The particles appear to have a velocity of 9 kilometers per second, and Gilmore believes that they interact with one another via some unknown force to spread out evenly. The nature of dark matter particles themselves remains one of the biggest mysteries of physics.

Scientists are reacting cautiously until they learn more about Gilmore's find, which he intends to submit to Astrophysical Journal. But the claim alone "will generate a lot of excitement," says cosmologist Robert Nichol of the University of Portsmouth, U.K. Mario Mateo of the University of Michigan, Ann Arbor, who also studies dwarf galaxies, was surprised by the density of dark matter Gilmore found. Still, he says, it's "pretty amazing" that although scientists can't see it or measure it, "we can start talking about constraining the nature of dark matter."

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