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The leading model of how the universe is structured says the biggest clusters shouldn’t be as distorted as they appear through telescopes.

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 is embedded within its own subhalo of dark matter.

For decades, astronomers have tried to understand how dark matter acts as the cosmos’ urban planner, shaping the structure of our universe. But the latest looks suggest that whatever dark matter is, it’s not behaving as researchers expected.

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