A team of astronomers from the University of Groningen has made a groundbreaking discovery: a gigantic, flat sheet of matter surrounding the Milky Way. This structure, dominated by dark matter, helps explain why most nearby galaxies are moving away from our galaxy rather than being drawn in by its gravitational pull. The findings, published on March 6, 2026, shed light on a longstanding mystery in cosmology.
For nearly a century, the work of astronomer Edwin Hubble established that the universe is expanding, with most galaxies receding from the Milky Way. Interestingly, the neighboring Andromeda Galaxy is an exception, as it approaches the Milky Way at a speed of approximately 100 kilometers per second. For about five decades, astronomers have grappled with the question of why many large galaxies, apart from Andromeda, appear to be moving outward instead of toward our galaxy, especially given the mass of the Local Group—comprising the Milky Way, Andromeda, and various smaller galaxies—that should exert a strong gravitational influence.
A New Understanding of Cosmic Structure
Led by PhD graduate Ewoud Wempe from the Kapteyn Institute, the international research team utilized advanced computer simulations to investigate the arrangement of matter surrounding the Local Group. Their findings reveal that this matter forms a broad, flattened sheet that extends for tens of millions of light-years. Importantly, this structure is not solely composed of ordinary matter but also includes the elusive dark matter that envelops galaxies.
Above and below this flattened region exist vast areas known as cosmic voids, which are largely empty. The simulations suggest that this specific arrangement of matter accurately reproduces both the positions and velocities of the galaxies observed in our cosmic neighborhood. Researchers describe the model created as a “virtual twin” of our cosmic environment, effectively replicating the dynamics of galaxies surrounding us.
To develop their model, the team began with conditions from the early universe, using measurements of the cosmic microwave background to estimate the initial distribution of matter shortly after the Big Bang. The computer simulations evolved this early universe forward, resulting in a system that matches the present-day Local Group’s configuration.
Revealing the Dynamics of Nearby Galaxies
The resulting simulations effectively replicate the masses, positions, and motions of both the Milky Way and Andromeda, as well as the locations and velocities of 31 galaxies just outside the Local Group. The model demonstrates that when the flat distribution of matter is included, the surrounding galaxies move away from the Milky Way at speeds consistent with actual observations.
Despite the gravitational influence of the Local Group, the galaxies within this plane are affected by additional mass distributed throughout the same region. This distant mass counteracts the gravitational pull of the Local Group. Consequently, the sparse distribution of galaxies in regions outside the plane accounts for the lack of objects falling toward us from those directions.
According to lead researcher Ewoud Wempe, this study marks a significant attempt to determine the distribution and motion of dark matter around the Milky Way and Andromeda. He 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 one hand, and with the dynamics of our local environment on the other.”
Fellow astronomer Amina Helmi expressed her enthusiasm for the findings, emphasizing that the research addresses a challenge that has puzzled scientists for decades. She noted, “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.”
This discovery not only enhances our understanding of the Milky Way’s cosmic surroundings but also contributes to the broader understanding of galaxy formation and dynamics in the universe.
