There’s something strange going on with the Milky Way. Recent measurements suggest that stars at the outskirts of our galaxy are misbehaving. They’re traveling far slower than similarly situated stars in other galaxies. One possible explanation for the Milky Way’s stellar slowpokes is that our galaxy is extraordinarily deficient in dark matter, the invisible substance thought to serve as gravitational scaffolding for cosmic structures. Another is that our core conceptions about dark matter—such as how much of it exists in the universe—are somehow deeply flawed.

Artist impression of ESA's Gaia satellite observing the Milky Way. The background image of the sky is compiled from data from more than 1.8 billion stars. It shows the total brightness and colour of stars observed by Gaia released as part of Gaia’s Early Data Release 3 (Gaia EDR3) in December 2020. Credit: Spacecraft: ESA/ATG medialab; Milky Way: ESA/Gaia/DPAC; (CC BY-SA 3.0 IGO); Acknowledgement: A. Moitinho. ADVERTISEMENT There’s something strange going on with the Milky Way. Recent measurements suggest that stars at the outskirts of our galaxy are misbehaving. They’re traveling far slower than similarly situated stars in other galaxies. One possible explanation for the Milky Way’s stellar slowpokes is that our galaxy is extraordinarily deficient in dark matter, the invisible substance thought to serve as gravitational scaffolding for cosmic structures. Another is that our core conceptions about dark matter—such as how much of it exists in the universe—are somehow deeply flawed. This head-scratcher stems from the European Space Agency’s Gaia satellite, which provides unparalleled information on the speeds and positions of nearly two billion stars in the Milky Way. Last year the Gaia team released the space-based telescope’s most precise measurements yet, spurring astronomers to refresh their galaxy-spanning assessments of stellar behavior. Several independent groups have now reported the oddly sluggish orbits of stars along the Milky Way’s outer rim, the peripheral edge of our galaxy’s luminous whorl. Stellar speeds offer a way to weigh a galaxy; the gravitational force each particular star feels depends on the galaxy’s total mass. A Gaia-derived study released on September 27 in the journal Astronomy & Astrophysics pegged the combined mass of our galaxy’s gas, dust, stars and dark matter at some 200 billion times that of our sun—hefty for you and me but on the order of five times less than that found in several other earlier assessments. Because the Milky Way’s visible material hasn’t disappeared, one easy—and especially thought-provoking—way to explain this result is that far less dark matter is floating around than previously believed.