The Milky Way contains an estimated 200 billion stars. But that’s just the bare tip of the iceberg—the Galaxy is surrounded by vast amounts of an unknown material called dark matter. Astronomers know it exists because, dynamically, the Milky Way would fly apart if dark matter didn’t keep a gravitational lid on things.

Still, astronomers would like to have a precise measure of the Galaxy’s mass to better understand how the myriad galaxies throughout the Universe form and evolve. A team of researchers from ESO, the Space Telescope Science Institute, the Johns Hopkins University Center for Astrophysical Sciences and the University of Cambridge combined observations from the NASA/ESA Hubble Space Telescope and ESA’s Gaia satellite to study the motions of globular star clusters that orbit our Galaxy.

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