for National Geographic News
There is a yawning gap of sky nearly a billion light-years across that contains no matter, a new study suggests.
But some researchers aren't buying it, in part because it would be a monumental surprise to find a void that large.
When seen on the scale of tens of millions of light-years, the universe has a foamy structure, with galaxies arranged as if on strings or sheets, with little matter in between them.
This arrangement applies to both visible matter that pumps out light, such as stars, and the mysterious dark matter, whose existence can be inferred only indirectly from how it holds galaxies together.
(Related: "'Cosmic Train Wreck' May Derail Theories of Dark Matter" [August 22, 2007].)
But at much larger scales, about 150 million light-years and beyond, researchers had expected the universe would be more uniform—so finding a void nearly a billion light-years across was a shock.
"Not only has no one ever found a void this big, but we never even expected to find one this size," said study lead author Lawrence Rudnick of the University of Minnesota in Minneapolis.
Rudnick and colleagues Shea Brown and Liliya Williams report their findings in a paper accepted for publication in the Astrophysical Journal.
Unusual Spot of Sky
The researchers began looking at this particular spot in the sky because it already showed a strange feature.
There the cosmic microwave background radiation—low-level light left over from the birth of our universe that bathes all of space—is especially dim.
This dark patch—where the sky appears "cooler"—is known as the "WMAP cold spot," named after the Wilkinson Microwave Anisotropy Probe satellite that mapped the radiation in 2003.
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