Cosmic void.

Astronomers have discovered a billion light-year-long region devoid of matter, the biggest hole in the cosmos they've yet observed.

A void this large hadn't been predicted by the leading model of cosmic evolution, says Lawrence Rudniek of the University of Minnesota in Minneapolis. He and his colleagues base their new findings on a radio-wavelength survey of 82 percent of the sky using the Very Large Array in Socorro, N.M. The dearth of galaxies lies in the constellation Eridanus, the team reports in an upcoming Astrophysical Journal.

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An earlier sky survey, based on data from NASA's Microwave Anisotropy Probe, yielded the intriguing result that the temperature of the cosmic-microwave background, me radiation left over from the Big Bang, is slightly lower in this patch of sky than in other regions.

Combining information from the two surveys, Rudnick and his colleagues suggest that the cool spot comes about because primordial radiation reaching Earth from that part of the sky has traveled through a region devoid of galaxies yet filled with dark energy.

First proposed by astronomers in 1998, dark energy makes the universe expand at an accelerated rate. In the absence of matter, dark energy causes photons from the microwave background to lose a tiny amount of energy, and therefore appear slightly cooler.

The team estimates that the void lies between 6 billion and 10 billion light-years from Earth.