There all along. In the solar neighborhood, clouds of cold, dark gas (yellow and red) appear to surround clouds of molecular hydrogen (blue).

Dark Matter Need Not Apply

The Milky Way galaxy may be held together by plain old ordinary matter after all. New research indicates that the space between stars contains twice as much cold gas as previously thought. If so, the gas might provide enough gravity to keep the galaxy from flying apart, eliminating the need for mysterious dark matter.

The matter strewn through interstellar space consists mostly of hydrogen. Nearly all of this comes in two forms: Relatively warm clouds of individual hydrogen atoms and colder clouds of diatomic molecules. Researchers measure the amount of warmer atomic hydrogen by tracking radio waves emitted by the atoms. But to spot the colder hydrogen, which emits no easily detectable signal, researchers must measure radio waves emitted by carbon monoxide and assume that the two molecules are found together in a certain ratio. Using such measurements, astrophysicists have estimated the amount of mass in our galaxy and come up with a value too small to generate enough gravity to prevent the outermost stars from whizzing into space. So researchers generally believe that some form of undetected dark matter must provide the extra gravity.

But much more cold gas lurks in our stellar neighborhood than previously believed, report Isabelle Grenier and colleagues from the University of Paris VII today in Science. The team compared a sky map of gamma rays generated by the Energetic Gamma Ray Experiment Telescope (EGRET) satellite to maps of atomic hydrogen, carbon monoxide, and dust within 1500 light years of earth. Gamma rays emanate from the clouds of dust and gas when cosmic rays slam into the matter within them. These dusty regions generate more gamma rays than expected, indicating that they contain vast quantities of cold gas that the radio studies had missed, the researchers report. Extrapolating from the size of the clouds of cold gas, the team estimates that the whole galaxy may contain twice as much ordinary matter as formerly believed. "It was a big surprise to me that we could miss that much" cold gas, Grenier says.

If the extrapolation is correct, the Milk Way may be hefty enough to hold itself together without any exotic dark matter, says Francoise Combes, an astrophysicist at the Observatory of Paris, although the mysterious stuff would still be needed to explain the formation of the earliest galaxies. But Don Cox, an astrophysicist at the University of Wisconsin, Madison, cautions that scaling up from the solar neighborhood to the entire galaxy is "kind of a long leap."

Related sites
The interstellar medium described at
Notes on dark matter
The EGRET web site