Slow-moving stars at the Milky Way’s outskirts suggest our galaxy may be far lighter than previously believed, with profound implications for dark matter

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The Gaia satellite, which was launched in 2013, offers the best-yet test of this simple notion via the spacecraft’s extraordinarily precise measurements of the three-dimensional positions and motions of stars in the Milky Way. But this testing has been a gradual process because the precision of Gaia’s reckoning improves in lockstep with how long it observes its stellar sample. Using Gaia, theoretical physicist Francesco Sylos Labini of the Enrico Fermi Study and Research Center in Italy and his associates saw subtle hints of a decline in the Milky Way’s stellar speeds a few years ago. Those hints became much more obvious in Gaia’s most recent data release, from 2022, which pegs stellar motions with twice the precision of a previous offering from 2018. Such improvements allow astronomers to plot the paths of stars with greater accuracy and out to much farther distances than before.

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