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Dark Matter Bridges Galaxy Clusters

Gravitational lensing shows that two galaxy clusters are connected by a filament of dark matter. John Matson reports

Dark matter is a big deal. The mysterious stuff makes up about a quarter of the universe, five times more than the ordinary matter of atoms and molecules. But we can’t see it. We can’t touch it. It’s just out there.

But it’s recognized by gravity. Big objects, such as galaxies, feel its gravitational tug. Galaxies such as our own Milky Way are believed to reside inside huge dark matter halos. And massive clusters of galaxies ought to be strung along filaments of dark matter, like pearls on a chain.

But those filaments are awfully hard to detect, being dark and all. Now a team of researchers reports identifying a dark matter filament joining two galaxy clusters. The astronomers used the technique called gravitational lensing to find the giant but invisible structure.

They measured how light from the distant universe gets rerouted slightly by massive objects, like it’s passing through a lens. Sure enough, there was an invisible bridge of massive stuff—dark matter—linking the galaxy clusters. The finding is in the journal Nature. [Jörg P. Dietrich et al., "A filament of dark matter between two clusters of galaxies"]

The filament is an important confirmation that dark matter is real. Now astrophysicists just need to figure out what it is.

—John Matson

[The above text is a transcript of this podcast.]

[Scientific American is part of Nature Publishing Group.]

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