For many years, scientists have tried to understand how the Milky Way is positioned in space, and how it moves together with nearby galaxies. Now, a new study has concluded that the Milky Way is not isolated in space, but embedded within a vast sheet of dark matter that stretches across millions of light-years. So, let’s find out more about our galaxy, shall we?
Galaxies in the universe
Galaxies are not spread in a uniform way throughout the universe. In some areas, they are together by forming clusters, in other parts of the universe they form long filaments, and there are also some places where nearly empty regions (voids) are found.
Inside this large cosmic scenario there is the Local Group, a region in the universe including the Milky Way, the Andromeda galaxy, and many smaller neighboring galaxies. Scientists have debated for decades how mass is distributed in this region and how that distribution affects the motion of nearby galaxies.
The Milky Way and the dark matter sheet
The new study was published in the scientific journal Nature Astronomy and its lead author is Ewoud Wempe, working with an international team of astronomers. The study reveals that the Milky Way is in a flat, sheet-like structure made mostly of dark matter. Don’t think this sheet is tiny, not at all. It extends beyond 10 megaparsecs, which is equal to about 30 million light-years. Above and below this sheet are regions of space that are almost empty.
This means that the mass surrounding the Local Group is not shaped like a sphere, as scientists once believed. Instead, it is concentrated within this flat structure. This new way of looking at the Milky Way’s environment changes how scientists understand our galaxy’s place in the nearby universe.
The role of dark matter
One of the key points of the study is the importance of dark matter, which doesn’t emit light and cannot be seen directly. Scientists know it exists because of the gravitational effects it has on visible matter.
The sheet described in the study is mainly formed by dark matter. The visible matter, like stars and gas, makes up only a small portion. The researchers found that the density at the center of the sheet is about twice the average density of the universe, and that the sheet has a thickness of about 1.6 megaparsecs. This structure helps explain why nearby galaxies are not falling rapidly toward the Local Group.
A model solving an old problem
For years, there was a contradiction between two ways of calculating the mass of the Local Group. On the one hand, the relationship between the Milky Way and the Andromeda would suggest strong gravitational attraction. On the other hand, observations showed that most nearby galaxies were moving away rather than falling inward.
The flat sheet model resolves this contradiction. It aligns with both local observations and the standard cosmological model (ΛCDM). It also explains why the local Hubble flow is so smooth, with very small deviations in galaxy speeds.
How the investigation was made
To reach the conclusions, the astronomy team used a simulation method called BORG (Bayesian Origin Reconstruction from Galaxies). This approach begins with the early conditions of the universe, based on the cosmic microwave background, and then simulates how the universe evolved over time.
The team created 169 independent simulations of a region similar to the Local Group. Each simulation included the Milky Way, Andromeda, and 31 nearby galaxies, with their positions and speeds closely matching real observations. In every simulation, the same result appeared: matter formed a dense, flat sheet, surrounded by nearly empty regions of space.
So, understanding that the Milky Way is part of this hidden framework brings us one step closer to understanding our cosmic neighborhood—and your place within it.