A closer look
Cosmology loves a good invisible mystery, especially when massive galaxies get into intergalactic pile-ups. When observing the Bullet Cluster—the aftermath of two gigantic galaxy clusters colliding—astronomers noticed something utterly bizarre. The normal, visible matter (mostly hot gas) physically slammed together in the center, dragging and heating up to millions of degrees. However, by measuring how gravity bent the light from background stars, scientists mapped the clusters' dark matter. They realized the massive dark matter halos hadn't slowed down at all; they had phased cleanly through each other without interacting, proving dark matter doesn't experience friction or electromagnetic forces.
