Most large galaxies near the Milky Way are racing away with the expansion of the universe, yet Andromeda is careering straight towards us. New research suggests our patch of the cosmos is shaped in a surprisingly flat way, and that this cosmic geometry explains why Andromeda is the odd one out.
A strange exception in a receding universe
For a century, astronomers have known that space itself is expanding. As it stretches, distant galaxies appear to recede from us, a pattern captured by Hubble’s law: the farther a galaxy is, the faster it seems to move away.
Andromeda refuses to play along. Sitting just 2.5 million light-years away, our nearest major galactic neighbour is hurtling towards the Milky Way at about 110 kilometres per second. At that speed, the two spirals are expected to collide and eventually merge in several billion years.
Yet most other hefty galaxies in our vicinity are not falling in with us. They are accelerating away, in some cases slightly faster than the expansion of space alone would predict.
The Milky Way and Andromeda are locked in a gravitational embrace, while almost every other big nearby galaxy is being tugged outward.
This mismatch has bothered cosmologists for decades. If our Local Group of galaxies is as massive as measurements suggest, its gravity should slow down nearby galaxies more than we actually see. Something else, beyond our familiar cluster, had to be reshaping the flow of matter.
Dark matter’s hidden role
The new study, published in Nature Astronomy, points the finger at dark matter — the invisible substance that outweighs normal matter in the universe by roughly five to one and interacts mainly through gravity.
Back in 1959, astronomers Franz Kahn and Lodewijk Woltjer had already argued that there must be extra unseen mass around the Milky Way and Andromeda for them to be on a collision course at all. That missing mass is now understood as dark matter, arranged in huge halos around each galaxy.
Those halos explain why Andromeda is moving towards us, but they cannot alone explain why other nearby galaxies seem to ignore the Local Group’s pull. The new work shows that the answer lies not just in how much dark matter is present, but in how it is arranged on scales of tens of millions of light-years.
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The team finds that mass around the Local Group is not roughly spherical, but squashed into a giant, thin sheet stretching far into space.
A flat sheet surrounding our galactic neighbourhood
To tackle the puzzle, researchers built detailed simulations of the “local universe” — a region extending roughly 32 million light-years from the Milky Way.
They started from the mass variations recorded in the cosmic microwave background, the faint afterglow left over from when the universe was only 380,000 years old. From that early map of tiny density ripples, they ran the clock forward, letting gravity do its work.
They demanded that the simulated universe recreate key features we see today:
- the masses of the Milky Way and Andromeda
- their present-day positions and velocities
- the locations and motions of 31 galaxies just outside the Local Group
Only when the mass outside the Local Group was shaped into a large, flattened sheet did the simulated galaxy motions line up with reality.
This sheet contains both dark matter and ordinary matter, but dark matter dominates. It stretches for tens of millions of light-years and seems to continue beyond the region the team simulated. The Milky Way, Andromeda and our smaller neighbours sit near the middle of this structure.
Why most galaxies are fleeing faster than expected
Galaxies just outside the Local Group are embedded within this vast dark matter sheet. That means they feel two competing gravitational tugs:
| Gravitational pull | Main effect on nearby galaxies |
|---|---|
| From the Local Group (Milky Way + Andromeda) | Pulls them inward, towards us |
| From the massive dark matter sheet | Pulls them outward, along the sheet, away from us |
Because so much mass sits in the sheet, slightly beyond our Local Group, its outward pull almost cancels the inward pull from the Milky Way and Andromeda.
The flattened mass distribution acts like a cosmic counterweight, allowing most nearby galaxies to keep sailing away, even in the face of our combined gravity.
As a result, galaxies within roughly 8 million light-years move away more slowly than a simple reading of Hubble’s law would suggest, while those beyond that distance are receding slightly faster than expected. The new simulations reproduce this break in behaviour naturally.
Cosmic voids carve out our safe zone
The sheet of matter is only half the story. Around it lie large, near-empty regions known as voids. These are places where the early universe had slightly lower density than average, so they expanded more quickly, thinning out over billions of years.
Our Local Group sits between such voids. Over cosmic time, their faster expansion has pushed matter from the low-density interiors into the denser “walls” between them — one of those walls being the sheet identified in the simulations.
Crucially, the regions above and below this sheet, in directions perpendicular to it, are almost completely devoid of galaxies. If there were galaxies living there, they would feel little pull from the dark matter sheet and would likely fall towards the Local Group instead.
The reason Andromeda is the only heavyweight galaxy racing towards us is simple: there are no other big galaxies sitting in the right place to do so.
In other words, the geometry of voids and walls around us has created a kind of safe corridor, where only Andromeda shares a strong mutual attraction with the Milky Way.
Building a universe that matches the data
The work also acts as a check on the standard cosmological model, often called “lambda cold dark matter”. That model combines cold (slow-moving) dark matter with dark energy, the mysterious driver behind cosmic acceleration.
By tuning their simulations to match the early universe and then comparing the result with current galaxy positions and velocities, the researchers tested whether lambda cold dark matter can produce a local environment like ours.
The fact that the best-fitting simulations both match observations and keep the standard cosmological model intact is a reassuring sign. The team can vary details such as initial conditions and still generate a Local Group and surrounding voids broadly similar to the ones we see on the sky.
What happens to the Milky Way next?
For anyone on Earth with a very long attention span, Andromeda remains the star of the future show. The galaxies are due to start merging in around 4 to 5 billion years. The night sky would eventually be filled with streams of stars as the two spirals tangle and blend.
Stars inside each galaxy are so far apart that direct collisions between individual stars will be rare. Instead, their orbits will slowly shuffle, and giant clouds of gas will crash together, sparking new waves of star formation.
The Sun will likely survive the event, though by that time it will be older, brighter and on its way to becoming a red giant. Any distant descendants of humanity might see the sky transformed as both galaxies fuse into a single, larger elliptical system.
Key concepts worth unpacking
What astronomers mean by “dark matter”
Dark matter is not simply normal matter that is hard to see. It does not emit or absorb light, and it does not match any known particle in the Standard Model of particle physics.
Astronomers infer its presence from its gravity:
- galaxies rotate too quickly in their outer regions for visible matter alone to hold them together
- light from distant galaxies is bent more strongly than visible mass would allow
- large-scale structures like sheets, filaments and clusters need extra mass to form on time
In this case, a vast flat “sheet” of dark matter, traced indirectly through galaxy motions, appears to shape the fate of the Milky Way itself.
What is the Hubble flow?
The Hubble flow is the general motion of galaxies as space expands. It is not galaxies flying through a static void, but the fabric of space between them stretching.
Local gravity can override this flow on small scales. The Milky Way, Andromeda and their small companions are gravitationally bound and move around a common centre of mass. Farther out, where the dark matter sheet’s tug and cosmic expansion dominate, galaxies mostly follow the Hubble flow with small deviations.
What future observations could reveal
The study suggests that galaxies located higher above or below the dark matter sheet should be falling towards it at hundreds of kilometres per second. These “infalling” systems provide a way to test the model.
As new surveys map the positions and velocities of ever more galaxies, especially in relatively nearby regions of the universe, astronomers will be able to check how well the predicted sheet, voids and incoming structures line up with reality.
Computer simulations will also grow more detailed. By varying assumptions about dark matter — for instance, whether it behaves exactly as “cold” and collisionless, or has subtle interactions — researchers can see how sensitive our local structure is to the underlying physics.
For now, the picture that emerges is unexpectedly elegant: the Milky Way is not at the centre of everything, but it does sit in a finely balanced patch of cosmic architecture. Most big galaxies are being shepherded away by a vast hidden sheet of dark matter, leaving just one massive neighbour, Andromeda, on a slow-motion collision course with us.