Dark matter halos (yellow) form around galaxiesRalf Kaehler/SLAC National Accelerator Laboratory
Delicate might not be the first word that springs to mind when you think of the Milky Way. But when Mariangela Lisanti started tinkering with the recipe for our galaxy, she found it surprisingly fragile.
Lisanti, a particle physicist at Princeton University, was simulating what would happen if dark matter – the mysterious stuff thought to account for over 80 per cent of all the matter in the universe – was more exotic than researchers typically assume. She swapped a small fraction of standard dark matter with something more complex. “We thought, we’re only adding 5 per cent, everything will be fine,” she says. “And then we just broke the galaxy.”
There is good reason for such meddling. Since the 1980s, astronomical signs have pointed towards dark matter being a single type of slow-moving particle that doesn’t interact with itself. Particle physicists have gone to great lengths to search for that particle. But decades later, it remains a no-show – perhaps because dark matter isn’t how we have tended to imagine it.

Recently, a series of galactic anomalies has sparked a scramble to explore alternatives. This “complex” dark matter might be as simple as sub-atomic particles that bounce off each other, or as complicated as families of dark particles that form dark atoms, stars and even galaxies. There is a daunting variety of possibilities.
But now, observations of anomalies in our galaxy finally promise to help us narrow down the options. And with…

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