Astronomers have known since 1957 that the Milky Way’s disc – where most of its hundreds of billions of stars reside – is not flat but somewhat curved upwards on one side and downwards on the other. For years, they debated what is causing this warp. They have proposed various theories including the influence of nearby galaxies or the effects of an imaginary dark matter halo. With its unique survey of more than one billion stars in our galaxy, ESA’s star-mapping satellite Gaia might hold the key to solving this mystery. A team of scientists using data from the second Gaia data release has now confirmed previous hints that this warp is not static but changes its orientation over time. Astronomers call this phenomenon precession and it could be compared to the wobble of a spinning top as its axis rotates.