Astronomers found the Milky Way lies within a massive flat sheet of dark matter about 32 million light-years across. Simulations show this structure and nearby cosmic voids balance gravitational forces, explaining why most nearby galaxies move away instead of falling toward the Local Group.Astronomers found the Milky Way lies within a massive flat sheet of dark matter about 32 million light-years across. Simulations show this structure and nearby cosmic voids balance gravitational forces, explaining why most nearby galaxies move away instead of falling toward the Local Group.