Like humans, galaxies can’t stand to be alone. Nudged by gravity, galaxies tend to group together, and some even end up in the universe’s equivalent of bustling megacities: clusters of up to a thousand galaxies, collectively outweighing our sun a million billion times over.

But for all the stars glimmering in these clusters, only a fraction of the entire structure’s mass is visible. As far as scientists can tell, a cluster’s real heft lies within a material that can’t be seen: an invisible, mysterious substance called dark matter. Like the concrete and asphalt beneath a city, a vast spherical halo of dark matter undergirds the whole cluster of galaxies. And just as buildings rise from the city streets, each individual galaxy