Disorientated matter (Image: David Law/University of Virginia)

The cloud of dark matter that is thought to surround the Milky Way may be shaped like a squashed beach ball. This halo of invisible matter also seems to sit at an unexpected angle – which could be a strike against a theory that challenges Einstein's account of gravity.

Dark matter is the stuff cosmologists invoke to explain why there appears to be far less mass in the universe than they think there should be. If they're right, the Milky Way is embedded in a vast halo of the stuff that is roughly 10 times as massive as all the galaxy's stars and gas combined. But the exact shape of this halo – which could bear traces of the collisions that built the galaxy – is still unknown.

To seek clues for how the dark matter is distributed, David Law of the University of California, Los Angeles, and colleagues studied the path of a shredded dwarf galaxy called Sagittarius, which fell into our galaxy more than 3 billion years ago. They reasoned that the tug of the Milky Way's dark matter should have influenced the trajectory of the stream of debris that formed as Sagittarius was torn apart.

The debris stream suggests the dark matter distribution is very different to that of ordinary matter, says Law. Instead of mimicking the Milky Way's disc of stars, as simulations had suggested, the halo is roughly perpendicular to the disc and is roughly half as thick as it is wide.

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