Gravitational lensing effects observed in galaxy clusters are stronger than explicable by visible matter alone.

Albert Einstein's theory of general relativity predicts that massive objects bend the fabric of spacetime, causing light to deflect as it passes by. This phenomenon, known as gravitational lensing, allows astronomers to 'weigh' galaxy clusters by observing how much they distort the light from more distant background galaxies. When scientists analyze these lensing effects, they consistently find that the gravitational pull exerted by galaxy clusters is far greater than what can be accounted for by the visible stars, gas, and dust. This discrepancy points to vast quantities of unseen mass within these clusters – dark matter – which contributes significantly to their overall gravitational potential. The stronger the lensing, the more unseen mass is inferred, providing a direct measurement of its gravitational influence independent of galactic rotation curves.