The Pancake We Float In

There is something deeply unsettling about not knowing the shape of your own home.

For nearly a century — ever since Edwin Hubble noticed that almost every galaxy in the sky is rushing away from us — astronomers have understood the broad strokes of cosmic expansion. The universe is inflating like a balloon, and galaxies ride its surface outward. Simple enough. But zoom in on our own cosmic neighbourhood, and things get strange.

Three Puzzles in One Backyard

The Milky Way lives in the Local Group, a modest club of about eighty galaxies dominated by us and Andromeda, our nearest large neighbour, which is barrelling toward us at roughly 100 kilometres per second. Beyond the Local Group, galaxies should feel our gravitational tug — the combined mass of the Milky Way and Andromeda is enormous. They should be slowing down, maybe even falling back.

They don't. They slide away with eerie smoothness, as if our gravity barely registers. Cosmologists call this the "quiet Hubble flow," and it has puzzled them for fifty years.

That's puzzle one. Here are the other two:

The Local Sheet. If you map the galaxies closest to us, they don't scatter in every direction. They arrange themselves in a curious flat plane — a pancake of galaxies stretching tens of millions of light-years. Why flat? Nobody was sure.

The Local Void. Adjacent to this plane sits an enormous pocket of almost nothing — a void so empty that galaxies appear to flee from it. The Milky Way itself seems to be receding from the Local Void at a speed that's hard to explain with gravity alone.

Three mysteries, all within our cosmic backyard. And now, in a paper published in Nature Astronomy, a team led by Ewoud Wempe of the University of Groningen's Kapteyn Institute has offered a single, elegant answer to all three.

A Virtual Twin of the Universe Next Door

Wempe and his collaborators — including Simon White, Amina Helmi, Guilhem Lavaux, and Jens Jasche — did something conceptually beautiful. They built a "virtual twin" of our cosmic environment.

Starting from conditions in the early universe (derived from measurements of the cosmic microwave background — that fading afterglow of the Big Bang), they ran sophisticated simulations forward through 13.8 billion years of cosmic evolution. But they didn't just simulate any random patch of space. They constrained their simulation to reproduce the specific masses, positions, and motions of the Milky Way, Andromeda, and 31 carefully selected galaxies just beyond the Local Group.

These 31 galaxies were chosen for their isolation — each one sitting far enough from massive neighbours that its motion traces the underlying architecture of space, undisturbed by local gravitational complications. Think of them as buoys on a dark ocean, revealing the currents beneath.

The result: the simulation only matched reality when the mass around us is arranged in a flat, sheet-like structure — a vast slab of matter, most of it dark, extending tens of millions of light-years across, with deep voids above and below.

The Dark Matter Pancake

Imagine the Local Group — our eighty galaxies, the Milky Way and Andromeda at the centre — sitting like blueberries in a cosmic pancake. The pancake is made mostly of dark matter, that invisible scaffolding that accounts for roughly 85% of all matter in the universe. Ordinary galaxies trace its shape, but the structure itself is far more massive than the galaxies alone.

This sheet does three things at once:

It explains the flat arrangement of nearby galaxies. They're flat because the underlying dark matter is flat. The Local Sheet isn't a coincidence — it's a consequence of the mass distribution.

It explains the Local Void. The sheet's gravity pulls matter out of the regions above and below, evacuating them into emptiness. The voids are carved by the pancake's gravitational appetite.

It explains the quiet Hubble flow. This is the cleverest part. Galaxies within the plane feel the gravitational pull of the Local Group, yes — but they also feel the pull of all the additional mass spread throughout the sheet beyond the Local Group. These two pulls roughly cancel each other out, leaving the galaxies to coast outward smoothly, as if our massive galactic partnership barely exists.

Fifty years of puzzlement, resolved by geometry.

No New Physics Required

What makes this result particularly satisfying is what it doesn't need. There's no exotic dark energy variation, no modified gravity, no new particles. The sheet fits comfortably within ΛCDM — the Lambda Cold Dark Matter model, which is the standard framework for understanding the universe's evolution. Sheets are a known feature of the cosmic web, the vast filamentary structure connecting galaxy clusters across the universe. We just didn't realise we were living in one.

"We are exploring all possible local configurations of the early universe that ultimately could lead to the Local Group," Wempe 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."

Amina Helmi, a co-author, added: "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."

Your Cosmic Address, Updated

We've known for a while that our cosmic address is layered: Earth orbits the Sun, which orbits in the Milky Way, which sits in the Local Group, which is part of the Laniakea Supercluster, which is a node in the cosmic web.

Now add another line: we live in a pancake.

It's a strangely comforting thought. The universe is enormous, chaotic, expanding in every direction — but right here, in our little corner, there's structure. A flat dark matter slab, tens of millions of light-years across, holding our neighbourhood together while voids yawn on either side. The quiet Hubble flow isn't a mystery anymore. It's a feature of the architecture.

We're not just drifting through space. We're embedded in it.

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The paper, "The mass distribution in and around the Local Group," by Ewoud Wempe, Simon D. M. White, Amina Helmi, Guilhem Lavaux, and Jens Jasche, is published in Nature Astronomy (2026).