Gravitational lensing is the bending of light from distant galaxies as it passes through the warped spacetime surrounding massive objects—and it has become the most direct method astronomers have for studying dark matter. Because dark matter neither emits nor absorbs light, it cannot be photographed. But its gravity bends passing light, and by measuring those distortions astronomers can reverse-engineer where the invisible mass lies. As an arXiv review of the field puts it, since direct detection experiments in underground mines have yet to find a convincing signal, "the best way to study dark matter is via its gravitational influence on more easily visible particles," with lensing the most direct method of all.
The Physics: Spacetime Curvature as a Cosmic Lens
Einstein's general relativity describes gravity not as a force but as the curvature of spacetime by mass and energy. Light follows the straightest available paths—geodesics—through this curved geometry, so when photons pass near a massive object, their trajectories are deflected. Matter, both ordinary and dark, therefore acts as "a kind of magnifying glass," in the words of the Euclid mission team at Caltech. The more massive and compact the intervening object, the stronger the deflection.
The consequences are visible in images of the deep universe. NASA explains that the gravity of intervening galaxy clusters and dark matter can smear or duplicate images of distant galaxies, or, when the alignment is just right, magnify them like a natural telescope. Crucially, the lensing effect depends only on mass, not on whether that mass shines—which is precisely why the technique works as a dark matter probe.
