A ‘large-scale sheet’ surrounding of the Milky Way explains the motion of nearby galaxies

Computer simulations carried out by astronomers from the University of Groningen in collaboration with researchers from Germany, France and Sweden show that most of the (dark) matter beyond the Local Group of galaxies (which includes the Milky Way and the Andromeda Galaxy) must be organised in an extended plane. Above and below this plane are large voids. The observed motions of nearby galaxies and the joint masses of the Milky Way and the Andromeda Galaxy can only be properly explained with this ‘flat’ mass distribution. The research, led by PhD graduate Ewoud Wempe and Professor Amina Helmi, was published in Nature Astronomy on 27 January.

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Almost a century ago, astronomer Edwin Hubble discovered that virtually all galaxies are moving away from the Milky Way. This is important evidence for the expansion of the universe and for the Big Bang. But even in Hubble's time, it was clear that there were exceptions. For example, our neighbouring galaxy, Andromeda, is moving towards us at a speed of about 100 kilometres per second.

In fact, for half a century, astronomers have been wondering why most large nearby galaxies – with the exception of Andromeda – are moving away from us and do not seem to be affected by the mass and gravity of the so-called Local Group (the Milky Way, the Andromeda Galaxy and dozens of smaller galaxies).

An international team of scientists led by PhD graduate Ewoud Wempe of the Kapteyn Institute in Groningen has found the solution to this puzzle. Computer simulations show that the mass distribution just slightly beyond the Local Group, including the invisible dark matter surrounding galaxies, is organised in a flat structure that extends tens of millions of light-years. Above and below this sheet are large voids. This solution, found by the computer, reproduces well the distribution and velocities of the observed galaxies around us.

A ‘virtual twin’ of the Local Group

The algorithm started from regions in the early universe, with a mass distribution based on observations of the cosmic microwave background. With a powerful computer, the model then evolved to reproduce the present-day characteristics of the Local Group, with the mass, position, and velocity of the Milky Way and the Andromeda Galaxy, and the positions and velocities of 31 galaxies just outside the Local Group. This resulted in the creation of simulations that may be considered ‘virtual twins’ of our cosmic environment.

In the computer result with the flat mass distribution, the 31 surrounding galaxies have a velocity comparable to that observed. Galaxies are moving away from us, despite the mass of the Local Group. The explanation is that for nearby galaxies in the plane, the gravitational pull of the Local Group is counteracted by the mass further away in the plane. And outside the plane, where you would expect matter to be moving towards us, there are no galaxies.

Pleased with the discovery

According to lead researcher Ewoud Wempe, this is the first assessment of the distribution and velocity of dark matter in the region surrounding the Milky Way and the Andromeda Galaxy. '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 is also very pleased with the discovery. According to her, astronomers have been trying to solve this problem for decades without success. '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.'

Reference: Wempe et al., The mass distribution in and around the Local Group, Nature Astronomy, 27 January 2026.