The collision of two galaxy clusters provides the best evidence yet of dark matter, the invisible substance theorized to dominate the mass of the universe. Images taken by the Chandra X-ray Observatory show that the larger cluster dragged out the bulk of the observable mass from the smaller one, in the form of hot plasma. According to theory, the collision should have separated the plasma from dark matter. Scientists determined how parts of the collided clusters bent light behind them via gravitational lensing—the more refraction, the greater the gravity in that area. They found that the plasma did not produce the strongest lensing, indicating that some other unseen mass was doing most of the bending. The latest results, described in the September 10 Astrophysical Journal Letters, do not indicate what the dark matter is, but they do undermine approaches that radically rework conventional gravitational theory.