Look up on a clear night and you’re staring into a paradox.
On the largest scales, the universe behaves like a rising loaf of bread: every raisin (galaxy) drifts away from every other raisin as space itself expands. This is the Hubble flow, and it’s one of the most consistent observations in cosmology. Distance up, speed away up.
Yet our closest large neighbor, Andromeda, refuses to cooperate.
Instead of receding, it’s heading straight for us at about 110 kilometers per second. For decades, that exception has been an odd footnote in astronomy textbooks: everything expands… except Andromeda. Now simulations suggest the explanation isn’t just a quirk of two galaxies tugging on each other. It’s the shape of the matter around us.
The simple expectation: everything should drift apart
Hubble’s law says galaxies move away at speeds proportional to their distance. Double the distance, double the recession speed.
So something 2.5 million light-years away should be receding, not approaching.
And yet Andromeda is inbound.
The usual explanation has been gravity: the Milky Way and Andromeda are massive enough to overcome expansion locally. Both sit inside large dark matter halos that add far more mass than their visible stars. That mutual attraction is strong enough to pull them together.
But this only explains the pair.
It doesn’t explain why most other nearby galaxies are moving away almost as if we weren’t there at all.
If gravity from our Local Group dominates the neighborhood, you’d expect many nearby galaxies to slow down or even fall toward us. They don’t.
The missing piece: the neighborhood isn’t spherical
Researchers ran detailed simulations of the local universe starting from early mass patterns seen in the cosmic microwave background. They then evolved the system forward, matching the known masses, positions, and velocities of the Milky Way, Andromeda, and dozens of nearby galaxies.
The surprising result: matter around us isn’t arranged in a roughly spherical cloud.
It’s flattened into a vast, sheet-like structure stretching tens of millions of light-years.
That geometry changes everything.
Nearby galaxies aren’t just feeling our pull. They’re embedded in this enormous, flat distribution of dark and visible matter whose gravity pulls outward, away from the Milky Way and Andromeda.
So for many galaxies:
Local Group gravity pulls inward The surrounding sheet pulls outward The forces nearly cancel
The result is that they continue following the Hubble flow, drifting away as if we barely mattered.
Andromeda is different because it’s already inside our shared gravitational well. It’s close enough that the mutual attraction dominates.
Why only one galaxy is coming at us
Imagine standing in a shallow valley surrounded by a broad plateau.
Objects on the plateau roll outward or sideways, not into the valley. Only objects already near the bottom move toward you.
That’s essentially our cosmic setup.
The Milky Way and Andromeda sit together in a local gravitational basin, wrapped in dark matter halos. Farther out, galaxies live in the flattened “wall” of mass. Above and below that sheet are low-density voids with very few galaxies at all.
So:
Galaxies in the sheet → pulled outward Galaxies in the voids → mostly absent Andromeda → already close and bound to us
Hence the lone inbound neighbor.
As Simon White put it, the flattened mass distribution “pulls these galaxies outwards in a way which almost exactly compensates for the inward pull of the [Milky Way] and [Andromeda].”
A reminder that gravity depends on shape, not just mass
What’s interesting here isn’t just Andromeda’s trajectory. It’s the lesson about structure.
We often talk about “how much mass” something has. But “where the mass is” can matter just as much.
A spherical distribution pulls differently than a sheet or a filament. The local universe turns out to be more like a cosmic wall than a blob.
That subtle geometry is enough to determine which galaxies drift away and which ones are destined to merge.
So Andromeda isn’t breaking the rules of expansion. It’s simply playing by a different set of local gravitational conditions.
If you want the full breakdown of the simulations and observations, you can read the coverage here:
Live Science – Every major galaxy is speeding away from the Milky Way, except one — and we finally know why
In the end, the exception makes sense: most galaxies are carried away by the large-scale flow, while one close neighbor sits in our shared gravitational pocket, slowly falling toward an eventual merger billions of years from now.