The Bullet Cluster provides direct observational evidence that dark matter is distinct from ordinary matter.

The Bullet Cluster (1E 0657-56) is a pair of colliding galaxy clusters, offering a unique opportunity to observe dark matter interacting – or rather, not interacting. When these two clusters smashed into each other at immense speeds, the ordinary matter (hot gas) slowed down due to electromagnetic friction and was clearly separated from the more diffuse dark matter. Observations using X-ray telescopes show that the bulk of the baryonic (normal) mass, in the form of hot gas, is concentrated in the center of the merged structure, glowing brightly in X-rays. However, gravitational lensing maps of the cluster reveal that the majority of the total mass (which includes dark matter) passed straight through each other, continuing along its original trajectory. This remarkable separation demonstrates that dark matter interacts with itself and with ordinary matter primarily through gravity, while electromagnetic and strong nuclear forces play little to no role, providing strong evidence against explanations that attribute 'missing mass' to modified gravity.