The Galaxy's Cosmic Exception: Why Andromeda Alone Is Rushing Toward Us

Every major galaxy astronomers can see is moving away from the Milky Way. There is one major exception to this rule: the Andromeda Galaxy. It is on a direct collision course with our own galaxy. For decades, this single exception has challenged a fundamental law of cosmic expansion. A new study now proposes an answer. It suggests a huge, flat sheet of invisible dark matter is pulling the other galaxies outward into deep space. This cosmic quirk is protecting the Milky Way from countless collisions. It also finally explains why Andromeda is the sole exception to the universal rule of recession.

"The observed motions of nearby galaxies and the joint masses of the Milky Way and the Andromeda Galaxy can only be properly explained with this 'flat' mass distribution," the researchers explained in a statement.

Future computer simulations based on this new model may further reveal how gravity has shaped our cosmic neighborhood.

The motion of galaxies through the expanding fabric of the universe is known as the Hubble flow. It is described by Hubble's law, named for astronomer Edwin Hubble. In the 1920s, Hubble discovered that the universe itself is expanding. His law describes a clear observational fact: galaxies are moving away from Earth at speeds that increase with their distance. The farther away a galaxy is, the faster it appears to be receding.

This makes the behavior of the Andromeda Galaxy a significant mystery. Andromeda is located 2.5 million light-years from Earth. Yet, it is speeding toward us at approximately 110 kilometers per second. Most other large galaxies near us are following the Hubble flow and moving away. Intriguingly, these receding galaxies appear to resist the powerful gravitational pull of our Local Group. The Local Group is a small cluster of galaxies bound by gravity, which includes the Milky Way, Andromeda, the Triangulum Galaxy, and dozens of smaller satellite galaxies.

This cosmic puzzle has persisted for more than half a century. In 1959, astronomers Franz Kahn and Lodewijk Woltjer found early evidence for what we now call dark matter around Andromeda and the Milky Way. They calculated that to reverse the initial expansion from the Big Bang and bring these two galaxies together, they would need a combined mass far greater than all their visible stars. We now know a huge portion of the mass of both galaxies is contained in vast, invisible halos of dark matter. These halos surround each galaxy and fuel their rapid approach toward one another.

However, this intense gravitational attraction does not seem to affect galaxies just outside the Local Group. "Material is actually moving away from the Milky Way faster than the Hubble flow," said study co-author Simon White, director emeritus of the Max Planck Institute for Astrophysics in Germany.

"Thus, galaxies closer than [roughly 8 million light-years] are moving away from us slower than predicted by Hubble's Law, whereas galaxies farther than [that] are actually receding faster than predicted," White explained.

To solve this puzzle, the research team built their own detailed digital universe. They ran numerous complex computer simulations. These models explored the interactions among dark matter, our Local Group, and the galaxies receding just beyond it. They looked out to a distance of approximately 32 million light-years from Earth.

The simulations modeled the evolution of our local universe from the beginning of space-time. They started with the initial mass distributions seen in the cosmic microwave background. This is the oldest light in the cosmos, emitted when the universe was only about 380,000 years old. The researchers then programmed the model to reproduce key characteristics observed in nearby galaxies. This included the mass, position, and velocity of Andromeda and the Milky Way. It also included the positions and velocities of 31 galaxies located just outside the Local Group.

This digital experiment revealed a critical finding. The mass just beyond the Local Group—which includes both invisible dark matter and visible matter—is not spread out evenly in all directions. Instead, it is concentrated into a vast, flat sheet or wall. This sheet stretches for tens of millions of light-years and continues even beyond the boundaries of the simulation.

Because nearby galaxies are embedded within this flattened sheet of matter, any inward gravitational pull from our Local Group is effectively canceled out. The stronger outward pull from the more distant mass within the sheet draws them away from us.

"If the mass were distributed approximately spherically around the Local Group, rather than being flat, then the external galaxies would be moving away from us slower than predicted," White said. "They would be slowed down by the gravitational pull of the Milky Way and Andromeda. Instead, the flattened distribution of the surrounding matter pulls these galaxies outwards. This almost exactly compensates for the inward pull of the Milky Way and Andromeda."

Equally important, the regions above and below this dense sheet of matter are mostly empty. They are cosmic voids. Such sparse regions are common throughout the universe. The deep voids around our Local Group formed in areas where the initial density of matter after the Big Bang was a bit lower than average.

"As a result these regions expanded faster than average, and their matter was 'pushed' outwards," White explained via email. "By the present day these low-density regions fill most of space. Gravitational effects have concentrated most of their material into the 'walls' that separate them." One of these dense walls is the flat sheet of matter that now surrounds our Local Group.

The location of these immense voids is essential to the story. These sparse regions are where any existing galaxies or structures would naturally fall toward the gravitational pull of the massive Local Group. Any galaxies located there would indeed be moving toward us. We do not see other large galaxies speeding toward the Milky Way like Andromeda because there are simply no other major galaxies in those nearby voids to do so.

When the simulations accounted for this vast, flat sheet of mass, they accurately reproduced the observed distribution of nearby galaxies and the surrounding voids. This finally reconciled long-standing experimental results with direct astronomical observations of how galaxies are moving. It also fit perfectly with the leading scientific model of the universe's structure and evolution, known as the lambda cold dark matter model.

"We are exploring all possible local configurations of the early universe that ultimately could lead to the Local Group," said lead study author Ewoud Wempe, a cosmologist at the University of Groningen in the Netherlands. "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."

Interestingly, the researchers note that some galaxies farther out in the cosmos have been observed falling toward this same flat sheet of matter. Finding additional galaxy structures falling from the directions of the voids could provide further support for the results of this study. The research paints a dynamic and intricate picture of our cosmic neighborhood, a structure shaped profoundly by the invisible influence of dark matter and the vast, empty expanses of cosmic voids.