Gravitational lensing effects observed around galaxy clusters directly map the distribution of dark matter.

Albert Einstein's theory of General Relativity predicts that massive objects bend spacetime, causing light from background sources to be deflected, much like a lens. This phenomenon, known as gravitational lensing, allows astronomers to 'weigh' objects, including those that are invisible. When light from distant galaxies passes through a massive galaxy cluster, it is distorted and magnified, creating arcs or multiple images of the background galaxies. A particularly compelling piece of evidence comes from observations of the Bullet Cluster (1E 0657-56), which is the result of two galaxy clusters colliding. Observations using X-ray telescopes show that the hot gas (ordinary matter) from the two clusters collided and slowed down, accumulating in the center. However, gravitational lensing measurements reveal that the bulk of the mass, including the dark matter, passed straight through each other, continuing along its original trajectory. This separation of visible baryonic matter from the dominant gravitational mass provides a direct and striking visual proof of dark matter's distinct, non-interacting nature.