On a clear night, the Milky Way and the Andromeda galaxy look like close neighbors. In space, they really are. Andromeda is even heading your way at about 100 kilometers per second. Now astronomers at the University of Groningen in the Netherlands say your galaxy neighborhood sits inside a much larger, pancake-like spread of matter, including dark matter you cannot see. The work comes from Ph.D. graduate Ewoud Wempe and Professor Amina Helmi at Groningen’s Kapteyn Institute, with collaborators in Germany, France and Sweden. Their study appears in Nature Astronomy. Various projections of the posterior mean density ρ of the constrained simulation ensemble, normalized by the cosmic mean density. (CREDIT: Nature Astronomy) A flatter neighborhood than expected For nearly a century, astronomers have known that most galaxies move away from the Milky Way as the universe expands. That trend is described by the Hubble-Lemaître law. But scientists have also struggled with a local mystery. Aside from Andromeda, many big nearby galaxies seem to drift away almost as if the Milky Way and Andromeda’s gravity barely …