The ballistics of galactic shrapnel show that the Milky Way has already crashed into its giant neighbour, Andromeda – but if that's right, physics is wrong

Double trouble: when galaxies collide Tatiana Plakhova

THE end of the Milky Way is already scheduled, and will be marked with fireworks. Some 4 billion years from now, the night skies will be lit by the glow of hundreds of billions of stars as the nearby Andromeda galaxy bears down on us. The two celestial giants will become one and stars, planets and gas clouds will be hurled into intergalactic space by titanic gravitational forces. Surviving stars and planets will be pitched into a jumbled cloud flaring up with new stars – floating into a long future not in the Milky Way, nor Andromeda, but a monstrous “Milkomeda” galaxy.

It’s a well-established picture of our galaxy’s cataclysmic future. More controversially, it might also be a vision of its past.

Observations indicate that the eviscerated remains of a past encounter between two celestial giants encircle our galaxy’s neighbourhood. Forbidden alignments of satellite galaxies, globular clusters and streams of stars trailing in our galactic wake all hint that our local cosmic history needs a rewrite. And not only that: to explain what our telescopes are telling us, we may need to rethink that most mysterious of substances, dark matter – and perhaps our entire conception of how gravity works, too.

Like many big problems, this one started out small: in a strange configuration of tiny dwarf galaxies surrounding the Milky Way. In 2012, astronomer Marcel Pawlowski, then of the University of Bonn in Germany, dubbed it the “vast polar structure“. This was for the way the dwarfs line up in a ring that circles the galaxy at right angles to the main disc of stars, which contains our sun …