In particle physics labs, like the Large Hadron Collider in Geneva, Switzerland, scientists smash atoms together to study the underpinnings of matter and energy. On the scale of the macrocosm, nature provides a similar experiment by crashing clusters of galaxies together. Besides galaxies and gas, the galaxy clusters contain huge amounts of dark matter. Dark matter is a transparent form of matter that makes up most of the mass in the universe. During collisions, the clouds of gas enveloping the galaxies crash into each other and slow down or stop. Astronomers found that the dark matter continued straight through the violent collisions, without slowing down relative to the galaxies. Their best explanation is that the dark matter did not interact with visible particles, and it also interacted less frequently with other dark matter than previously thought. Astronomers used the Hubble Space Telescope and Chandra X-ray Observatory to study 72 large galaxy cluster collisions. Chandra traced the hot gas, and Hubble saw how the invisible dark matter warps space and distorts the images of background stars. This allowed for the distribution of dark matter in the collision to be mapped. The finding narrows down the options for what this dark matter might be.