Most matter in the universe is dark matter, made up of as-yet-unidentified particles that don’t interact electromagnetically with the baryonic matter we’re familiar with. It doesn’t emit, absorb, or scatter radiation at any wavelength. But, like ordinary matter, it does exert a gravitational influence on photons, deflecting their paths as they travel over cosmic distances. That effect, called gravitational lensing, has been used to map the large-scale structure of dark matter through distortions in images of background galaxies (see PHYSICS TODAY, March 2007, page 20). But a more complete picture may be available from gravitational lensing of the cosmic microwave background.

Lensing of the CMB has been observed before, using data from the Wilkinson Microwave Anisotropy Probe (WMAP).1 But the only conclusive demonstrations of lensing from WMAP data involved cross-correlations between the CMB data and observations of foreground galaxy clusters. Because baryonic matter and dark matter...

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