For nearly a century, astronomers have known something strange about our corner of the universe.

Most galaxies around the Milky Way are drifting away. That part fits with what scientists expect in an expanding universe. But here’s the odd part. These galaxies sit close to the Local Group, a collection of galaxies whose gravity should be strong enough to pull many of them inward.

Yet they keep moving away.

Now, researchers think they finally know why.

Nearly 100 years ago, astronomer Edwin Hubble discovered that most galaxies are receding from the Milky Way. His observation became a foundation of modern cosmology because it showed that the universe is expanding and supports the idea that everything began with the Big Bang.

Even then, astronomers noticed exceptions. The most famous is the Andromeda Galaxy, our nearest large neighbor, which is actually heading toward the Milky Way at about 100 kilometres per second.

But Andromeda is not the only curiosity. For decades, scientists have struggled to explain another puzzle. Most other large galaxies close to the Local Group appear to be moving away from us rather than being pulled inward by gravity.

The Local Group includes the Milky Way, Andromeda and dozens of smaller galaxies. Together, they contain a massive amount of matter. That mass should exert a noticeable gravitational pull on nearby galaxies.

Instead, many of them seem to be escaping it.

A new study led by PhD graduate Ewoud Wempe of the Kapteyn Institute in Groningen suggests the answer lies in how matter is arranged in the region around us.

Using advanced computer simulations, the international research team found that the material surrounding the Local Group forms a vast, flattened sheet stretching tens of millions of light-years across.

This cosmic sheet includes both normal matter and dark matter, the invisible substance believed to make up much of the universe’s mass.

Above and below this flattened region sit enormous empty spaces known as cosmic voids.

When the scientists built simulations using this structure, the results closely matched what astronomers actually observe. The virtual universe reproduced both the positions of nearby galaxies and the speeds at which they move.

To create the model, researchers began with conditions from the early universe. They used measurements of the cosmic microwave background, the faint radiation left over from the Big Bang, to estimate how matter was distributed shortly after the universe formed.

A powerful computer then evolved that early universe forward through billions of years, eventually producing a system that resembles the present-day Local Group.

The simulation recreated the masses, locations and motions of the Milky Way and Andromeda, along with 31 galaxies just outside the Local Group.

Because the model mirrors our surroundings so closely, researchers describe it as a “virtual twin” of our cosmic neighbourhood.

The flattened sheet of matter plays a key role in explaining the strange galaxy motions.

Galaxies within this plane are influenced not only by the gravity of the Local Group but also by additional mass spread throughout the same plane. That distant mass helps counterbalance the Local Group’s gravitational pull.

At the same time, the cosmic voids above and below the sheet contain very few galaxies, which explains why astronomers do not see many objects falling toward us from those directions.

According to Wempe, the research marks an important step in understanding how dark matter shapes the region around our galaxy.

“We are exploring all possible local configurations of the early universe that ultimately could lead to the Local Group,” he said. “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.”

Astronomer Amina Helmi said the findings help resolve a question that has lingered for decades.

“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.”

For scientists studying the structure of the universe, that connection between motion and invisible matter is a powerful clue.

It suggests that the unusual behaviour of nearby galaxies may not be unusual at all. It may simply reflect the vast cosmic structure we happen to live inside.