Gravitational Lensing

One of the strongest lines of evidence for dark matter is gravitational lensing. Massive objects warp spacetime, bending light that passes near them. Galaxy clusters distort the light of more distant galaxies behind them, creating arcs and multiple images.

The amount of bending reveals the total mass - and it far exceeds the visible matter.

The Bullet Cluster provides the most compelling evidence. Two galaxy clusters collided head-on. The hot gas — most of the ordinary matter, glowing in X-rays — dragged against itself and slowed in the collision zone. But gravitational lensing shows the bulk of the mass sailed straight through and now sits well ahead of the gas. Mass separated from ordinary matter, exactly as dark matter that barely interacts would behave.