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 revealed the large-scale structure of dark matter through distortions in images of background galaxies. But a more complete picture may be available from the cosmic microwave background (CMB), the ubiquitous thermal radiation that originated 380 000 years after the Big Bang. Until recently, gravitational lensing of the CMB could be conclusively demonstrated only through cross-correlations with maps of foreground galaxy clusters, which serve as tracers of where the dark matter is expected to be. But now, using high-resolution data from the NSF-funded Atacama Cosmology Telescope (shown in the photograph), researchers have detected CMB lensing from CMB data alone. The team’s most recent result is a statistical determination that the CMB has been gravitationally lensed. But the researchers anticipate that as better CMB data become available, it will be possible to reconstruct the lensing field and map the lensing structures. Furthermore, they’ve used their data to demonstrate the need for dark energy—the elusive energy that permeates all space and acts to speed up the expansion of the universe—the first such demonstration using only CMB data. (S. Das et al., Phys. Rev. Lett. 107, 021301, 2011 ; B. D. Sherwin et al., Phys. Rev. Lett., 107, 021302, 2011 .)—Johanna Miller
