Astronomers have discovered that the Milky Way resides within a vast, flat sheet of matter dominated by dark matter, surrounded by enormous empty voids. This structure explains why most nearby galaxies are moving away from our Local Group rather than being drawn in by gravity. The finding, based on advanced simulations, resolves a longstanding puzzle in cosmology.
Nearly a century ago, Edwin Hubble observed that most galaxies are receding from the Milky Way, supporting the idea of an expanding universe originating from the Big Bang. However, Andromeda stands out as an exception, approaching at about 100 kilometers per second. For the past 50 years, researchers have been puzzled by the outward motion of other large galaxies near the Local Group, which includes the Milky Way, Andromeda, and smaller companions, as their combined gravity should pull these neighbors closer rather than repel them. An international team, led by PhD graduate Ewoud Wempe from the Kapteyn Institute in Groningen, used computer simulations to investigate. Starting from early universe conditions derived from cosmic microwave background measurements, they evolved a model forward in time to replicate the current Local Group. The simulations revealed a broad, flattened distribution of matter spanning tens of millions of light-years, encompassing both ordinary and dark matter. This cosmic sheet is flanked above and below by vast cosmic voids with minimal matter. The model accurately reproduces the masses, positions, and velocities of the Milky Way, Andromeda, and 31 nearby galaxies outside the Local Group, earning it the description of a 'virtual twin' of our cosmic neighborhood. Within this plane, the spread of mass counteracts the Local Group's gravitational pull, allowing galaxies to drift outward, while the sparse voids explain the lack of inward motion from other directions. Wempe stated, 'We are exploring all possible local configurations of the early universe that ultimately could lead to the Local Group. It is great that we now have a model that is consistent with the current cosmological model on the one hand, and with the dynamics of our local environment on the other.' Amina Helmi, an astronomer, added, 'I am excited to see that, based purely on the motions of galaxies, we can determine a mass distribution that corresponds to the positions of galaxies within and just outside the Local Group.' The research, published in Nature Astronomy in 2026, marks the first detailed mapping of dark matter distribution around the Milky Way and Andromeda.