If the concept of dark matter gives you a bit of a headache, hold on to your Advil.
Theorists attempting to explain some of the "missing mass" in the universe now say there may be entire galaxies that are dark. The new idea, proposed by Neil Trentham of the University of Cambridge, along with colleagues Ole Moller and Enrico Ramirez-Ruizof, suggests that for every normal, star-filled galaxy, there may be 100 that contain nothing, or at least nothing that we understand.
This so-called dark matter -- which may or may not be composed of the same particles that form you, me and the fence post -- would in this scenario have created numerous low-mass mini-galaxies that lurk in what is otherwise known as intergalactic space.
Trentham explains that many large galaxies are known to be accompanied by smaller galactic partners. But in some instances, as many as
"Hence the expectation that many dark galaxies exist," Trentham said in an e-mail interview, suggesting that "very few stars -- perhaps none at all -- have formed in them."
A paper on the idea will be published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.
Cautious response
Alan Dressler of the Carnegie Institution of Washington said dark mini-galaxies may well exist, but don't expect a bunch of undiscovered
to be hanging out in space. Dark galaxies would be a hundred or even a thousand times smaller than our own galaxy.[inset]
Dressler, who was not involved in the new work, studies galaxy formation and evolution. His work led to the discovery of the "great attractor," a huge concentration of galaxies and invisible matter that seems to tug at our Milky Way galaxy.
"The bulk of the dark matter [in the universe] is probably associated with the [known and detectable] bright galaxies, and is known to be there." Dressler told SPACE.com. "But this is still an interesting hypothesis that for the dark mini-galaxies, the dark-matter substructure does not have stars accompanying it in all cases."
Dressler said it would be fascinating to learn that there are more dark mini-galaxies in the universe than there are larger, normal galaxies. "By number, I'll bet there very well could be," he said, stressing that their mass would not be terribly significant, however.
Morris Aizenman, of the National Science Foundation's and Physical Science Directorate, called the new idea intriguing, but he cautioned that more study is needed to determine its merits.
"Everyone agrees that there is a significant amount of dark matter in most galaxies," Aizenman said. "But the real question is why a galaxy would be composed entirely of dark matter."
Dark matter was first suggested six decades ago as a way of explaining curious motions in the cosmos. Figuring out what it is, and exactly how much exists, would be an important step toward understanding how galaxies form and whether or not the universe will continue expanding forever.
Though not yet seen and not even proved to exist, dark matter gives itself away by gravitationally altering the movement of galaxies and stars. For example, matter near the center of most galaxies moves so fast that some unseen mass must exist at the center, pulling things inward. And even the path of light from distant objects is skewed by material that cannot be found.
In large clusters of galaxies, five to possibly 10 times more matter exists than can be accounted for by the stars and gas scientists can find.
Next page: A possible smoking gun.
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