Astronomers have untangled a messy collision between two massive clusters of galaxies in which the clusters' vast clouds of dark matter have decoupled from the so-called normal matter. The two clusters each contain thousands of galaxies and are located billions of light-years away from Earth. As they plowed through each other, the dark matter-an invisible substance that feels the force of gravity but emits no light-sped ahead of the normal matter. The new observations are the first to directly probe the decoupling of the dark and normal matter velocities.
Galaxy clusters are among the largest structures in the universe, glued together by the force of gravity. Only 15 percent of the mass in such clusters is normal matter, the same matter that makes up planets, people, and everything you see around you. Of this normal matter, the vast majority is hot gas, while the rest is stars and planets. The remaining 85 percent of the cluster mass is dark matter.
During the tussle that took place between the clusters, known collectivity as MACS J0018.5+1626, the individual galaxies themselves largely went unscathed because so much space exists between them. But when the enormous stores of gas between the galaxies (the normal matter) collided, the gas became turbulent and superheated. While all matter, including both normal matter and dark matter, interacts via gravity, the normal matter also interacts via electromagnetism, which slows it down during a collision. So, while the normal matter became bogged down, the pools of dark matter within each cluster sailed on through.
