How do scientists primarily infer the existence of dark matter?

Scientists primarily infer the existence of dark matter by observing gravitational effects on visible matter. This inference stems from the observed behavior of galaxies and galaxy clusters, where the visible mass—derived from stars, gas, and dust—does not correspond to the amount of gravitational force needed to hold them together. For instance, when observing spiral galaxies, the rotation speeds of the outer stars are much higher than what the mass of the visible matter would dictate. If only visible matter were present, these outer stars would be moving more slowly and could even escape the galaxy's gravitational pull.

Additionally, studies of galaxy clusters reveal that they contain far more mass than what can be accounted for by the visible matter alone. The gravitational lensing effect, where the light from distant objects is bent around massive structures, also provides strong evidence for unseen mass—consistent with the presence of dark matter.

These findings illustrate that dark matter does not emit light or energy that can be directly detected, thus making gravitational interactions the primary method through which scientists infer its existence.