A big find in the hunt for elusive dark matter
Astronomers cite new evidence that the unseen 'glue' holding galaxies together really exists.
By Peter N. Spotts | Staff writer of The Christian Science Monitorfrom the May 16, 2007 edition
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Building cosmic maps of matter no one can see may seem like a strange way to make a living. But for astronomer Myungkook James Jee, the work may have netted evidence that seals the case for the existence of dark matter – unseen, "missing mass" first invoked in 1933 to explain why spinning galaxies and galaxy clusters don't tear themselves apart as they twirl.
Dr. Jee and his colleagues announced Tuesday that they have discovered a wispy ring of dark matter around a huge cluster of galaxies 5 billion light-years from Earth. The researchers didn't spot the dark matter directly; it doesn't emit light. Instead, they analyzed the way its gravity bends light from galaxies behind it – an effect called gravitational lensing. If confirmed, the result would rule out other, more exotic notions that have emerged to explain how galaxies and galaxy clusters are held together.
Typically, dark matter's gravity is thought to act as a scaffold for building galaxies and structures that astronomers can detect. That was their only initial explanation for how these structures held up – otherwise the mass in luminous matter wouldn't yield enough gravity to hold galaxies or galaxy clusters together as they rotate. So, astronomers surmised, bright and dark matter generally should go hand in glove.
Some scientists, however, are uncomfortable relying on the unseeable to act as this cosmic glue. For nearly 25 years, they have sought other ways to explain the structures they see. For example, some suggest that you don't need to invoke dark matter if gravity behaves differently at huge spatial scales than it does at small scales. Such alternative explanations couldn't be ruled out – unless someone could find dark matter out there on its own, with a structure unlike that of the nearest clusters.
Jee's discovery gets closer than ever to doing just that. It suggests that the ring of dark matter, 2.6 million light-years wide, is clearly separated from the hot gas and other luminous matter in the cluster. If the result holds up, this stand-alone dark matter would close the door to these other theories, some astronomers say.
It would also sustain a long-cherished assumption underlying space science: Gravity works the same way everywhere in the universe as it does on Earth.
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