Nearly a century ago, astronomer Edwin Hubble discovered that almost all galaxies are receding from the Milky Way. This observation became a cornerstone of modern cosmology because it provided key evidence that the universe is expanding and that it began with the Big Bang. Even during Hubble's era, however, astronomers knew the pattern was not universal. One notable exception is our neighboring galaxy Andromeda, which is moving toward the Milky Way at roughly 100 kilometers per second.
For about fifty years, scientists have puzzled over another related mystery. Most large galaxies near our own, aside from Andromeda, appear to be moving away from us rather than being pulled inward by gravity. This seems surprising because these galaxies reside near the Local Group (the Milky Way, the Andromeda Galaxy and dozens of smaller galaxies), whose combined mass should exert a noticeable gravitational influence.
An international research team led by PhD graduate Ewoud Wempe of the Kapteyn Institute in Groningen believes it has found the explanation. Using advanced computer simulations, the researchers discovered that the matter surrounding the Local Group is arranged in a broad, flattened structure that stretches tens of millions of light-years across. This structure includes not only ordinary matter but also the invisible dark matter that surrounds galaxies. Above and below this flattened region lie enormous empty areas known as cosmic voids.
The simulations show that this arrangement of matter can accurately reproduce both the positions and speeds of the galaxies observed around us. In other words, the computer model successfully recreates the same patterns astronomers see in the real universe.
To build their model, the scientists began with conditions from the early universe. They used measurements of the cosmic microwave background to estimate how matter was distributed shortly after the Big Bang. A powerful computer then evolved this early universe forward in time, eventually producing a system that matches the present day Local Group.
The resulting simulations replicate the masses, locations, and motions of the Milky Way and Andromeda, as well as the positions and velocities of 31 galaxies just outside the Local Group. Because the model so closely resembles our surroundings, researchers describe it as a "virtual twin" of our cosmic environment.
When the model includes the flat distribution of matter, the surrounding galaxies move away from us at speeds similar to those actually observed. Despite the gravitational pull of the Local Group, galaxies within the plane are influenced by additional mass spread throughout that same plane. This distant mass counterbalances the Local Group's gravity. Meanwhile, regions outside the plane contain very few galaxies, which explains why we do not see objects falling toward us from those directions.
According to lead researcher Ewoud Wempe, the study represents the first detailed attempt to determine the distribution and motion of dark matter in the area around the Milky Way and Andromeda. "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."
Astronomer Amina Helmi also welcomed the findings, noting that the problem has challenged researchers 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."
