An international team of astronomers from the University of Groningen, as well as from Germany, France, and Sweden, has shown that most of the matter outside the Local Group of galaxies is not arranged chaotically, but instead forms a huge flat structure, a “sheet” tens of millions of light-years across. The study was published in the journal Nature Astronomy.

Why Our Neighbors Are “Running Away” from the Milky Way

The Local Group includes the Milky Way, the Andromeda galaxy, and several dozen dwarf galaxies. For almost a hundred years it has been known that nearly all large neighboring galaxies, except Andromeda, continue to move away from us, following the Hubble–Lemaître law. This seemed strange, because the gravity of the Local Group should have been strong enough to pull them back.

A team led by Ewoud Wempé and Amina Helmi decided to investigate this paradox. They built extremely precise computer simulations, starting from the conditions of the early Universe based on data from the cosmic microwave background and then evolving this region forward to the present day. The model accurately reproduced the masses, positions, and velocities of the Milky Way, Andromeda, and 31 galaxies beyond them.

The Structure of the “Sheet” and the Voids

It turned out that all matter around the Local Group, including invisible dark matter, forms a gigantic flat structure. Above and below this plane lie enormous voids that are almost completely devoid of galaxies.

The gravitational pull of the Local Group on galaxies lying in the plane of the sheet is balanced by the mass distributed farther away within the same sheet. In the directions from which matter would be expected to fall toward us, there are simply no galaxies, only empty space. This is why neighboring galaxies continue to move away, following the general expansion of the Universe.

The First Precise Model of the Local Cosmic Environment

According to Ewoud Wempé, this is the first study that simultaneously agrees with modern cosmology and with the real dynamics of our immediate cosmic neighborhood.

“We managed to obtain a model that is consistent both with modern cosmology and with the real dynamics of our local environment,” he noted.

In Short

Scientists have shown that the dark matter around the Milky Way and Andromeda is organized into a gigantic flat “sheet” tens of millions of light-years in size. Above and below this plane lie huge voids. This structure explains why neighboring galaxies continue to move away from the Local Group despite its mass. The model accurately reproduced the real velocities and positions of 33 galaxies, marking the first time the local cosmic environment has been described in such detail. The discovery changes our understanding of the nearest part of the Universe.