Dark Matter and Normal Matter Decouple

This artist’s animation depicts a collision between two massive clusters of galaxies. As the collision progresses, the dark matter in the galaxy clusters (blue) moves ahead of the associated clouds of hot gas, or normal matter (orange). This happens because, while both dark matter and normal matter feel the pull of gravity, only the normal matter experiences additional effects like shocks and turbulence, which slow it down during the collision. In this animation, the clusters are pictured in an orientation similar to that of the well-known Bullet Cluster collision, where the separation of dark matter and normal matter is observed as a spatial offset. From our view on Earth, MACS J0018.5 is in fact rotated nearly 90 degrees relative to the Bullet cluster and from what is depicted here. In other words, the two massive clusters in MACS J0018.5 are positioned such that one is flying toward us, and the other is flying away. This unique perspective allowed researchers to measure velocity differences between the dark matter and normal matter in a cluster collision for the first time. Image credit: W.M. Keck Observatory/Adam Makarenko More info: