A decade ago cosmologists began to suspect that the universe might be bizarrely lopsided. Hints of a universal imbalance emerged from the afterglow of the big bang, known as the cosmic microwave background, or CMB, which is dotted with hot and cold spots signifying fluctuations in the density of matter. Starting in 2003, data from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) suggested that one side of the cosmos is hotter than the other. But the finding rubbed against the prevailing view in cosmology that the universe expanded titanically during an early growth spurt called inflation, which should have left the CMB looking mostly uniform.

In recent months the case for lopsidedness has gotten much stronger—the European Space Agency's Planck satellite, which is newer and more sensitive than WMAP, has returned similarly reliable evidence of an asymmetric cosmos. The question now is whether the enigma demands a cosmic rethink or whether it results from an extremely unlikely—but ultimately explainable—occurrence.

“After quite a few years of claims based on independent researchers' analyses of publicly available WMAP data, we now have redundancy from Planck as convincing support,” says cosmologist Krzysztof Gorski of the NASA Jet Propulsion Laboratory in Pasadena, Calif.

The surprising temperature difference may have become more believable, but it remains a puzzle. Cosmologist Yabebal Fantaye of the University of Oslo, along with Gorski and others, recently ran 10,000 simulations of how the CMB should look, given the Standard Model of the universe's evolution. Only seven outcomes resembled the picture that WMAP has assembled, the researchers reported in Astrophysical Journal Letters. In other words, the standard cosmological paradigm can accommodate a lopsided universe but just barely. “It certainly could happen, but it is not very likely,” Fantaye says.

Researchers are already exploring the possibility that the asymmetry points to something new—whether hypothetical energy fields warping the newborn universe or ancient bruises from collisions with another universe. Further clues will arrive next year when the Planck team releases data on the polarization of the CMB—the way its photons oscillate—which could distinguish between such cosmological exotica and a mere quirk that fits within the prevailing paradigm.