How does gravitational lensing reveal dark matter in galaxies and galaxy clusters?

Gravitational lensing is a phenomenon where massive objects, such as galaxies or galaxy clusters, bend space and distort the light passing near them from more distant background objects. Because dark matter exerts a gravitational pull despite being invisible, it contributes significantly to this bending effect.123

Astronomers use this process to reveal dark matter through two primary methods:

• Strong Lensing: When a lens is extremely massive, it can create distinct multiple images, rings, or highly stretched arcs of background objects. By analyzing these distortions, scientists can calculate the total mass required to produce such effects, which often far exceeds the mass of visible matter.1
• Weak Lensing: Often, the intervening mass is only strong enough to slightly stretch or distort the shapes of distant galaxies. By statistically analyzing the shapes of many background objects, astronomers can map the distribution of dark matter within galaxies and clusters.12

This technique provides critical evidence that dark matter is a physical reality, as it allows researchers to map mass that does not emit or interact with light.32