Our home galaxy has been weighed, and it is surprisingly lean. The latest gauge of the dark matter mass of the Milky Way suggests it weighs only a quarter to a third of the amount previously estimated. This could explain the dearth of smaller galaxies buzzing around the Milky Way. But it also means we may live in a cosmic anomaly.
It is thought the first galaxies were born as normal matter coalesced around globs of dark matter, the invisible stuff thought to make up about 80 per cent of the matter in the universe. We can't see dark matter itself, but we can trace its effects in the motions of stars in modern galaxies.
Stars on the edges of large spirals like the Milky Way are orbiting so fast that they should fly off, so something must be holding on to them. That thing is thought to be a halo of dark matter encircling the visible disc.
Knowing our galaxy's total mass will tell us a lot about it. "Is our Milky Way typical, or is it actually quite weird?" asks Alis Deason of the University of California, Santa Cruz.
A smattering of stars live in the Milky Way's dark matter halo, and previous studies have used their motion to figure out the halo's mass. But we are embedded in a spiral arm, which means dust and gas blocks much of our view of our relatively flat galaxy, so those models had to make assumptions about the parts we can't see.
To get around the uncertainties, Deason and her colleagues compared two supercomputer simulations that mix different amounts of normal and dark matter to build the Milky Way.