Dark matter, the substance no one has ever seen, continues to baffle cosmologists. New observations of the distribution of dark matter in a distant cluster of galaxies may even force scientists to propose a fifth force of nature—or to rewrite the basics of Newtonian gravity. Little wonder that many researchers hope that the unsettling result will turn out to be an observational fluke.

Giant clusters of galaxies consist of two observable components. The galaxies themselves can be seen with large optical telescopes. The hot, tenuous gas in between the galaxies can be spied and mapped by x-ray satellites such as NASA’s Chandra X-ray Observatory. But according to current wisdom, a galaxy cluster may have a third, invisible component: mysterious dark matter that pulls in the atoms from which stars and galaxies form. Dark matter can be charted only by the subtle way its gravity slightly bends light, altering the shapes of faint galaxies in the distant background. The ability to detect such “weak lensing” has seen major improvements during the past decade.

This article was originally published with the title Dark Riddles.

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