The Milky Way's Cosmic Floor: We Live on a 30-Million-Light-Year Sheet of Dark Matter

Abstract

For nearly a century, the motion of the Milky Way and its neighbors has presented a paradox to cosmologists. The local universe appears dynamically "cold," with galaxies drifting calmly with the cosmic expansion, yet the high mass of the Local Group suggests a violent, chaotic history that should have disturbed this peace. In early 2026, a groundbreaking study published in Nature Astronomy by Ewoud Wempe and collaborators resolved this tension by revealing a hidden architectural marvel: the Milky Way is not floating in a featureless void or a simple spherical halo, but is embedded within a massive, flattened "sheet" of dark matter. Extending over 30 million light-years, this structure exerts a unique gravitational influence that stabilizes our neighborhood. This report explores the discovery of the "Local Dark Matter Sheet," detailing the advanced Bayesian simulations that brought it to light, the specific dynamics that allow for a "cold" Hubble flow amidst high mass, and the broader implications for the standard model of cosmology. 

1. Introduction: The Quiet Neighborhood Paradox

In the grand scale of the cosmos, the Milky Way resides in a bustling metropolitan area known as the Local Group, dominated by our galaxy and its massive neighbor, Andromeda (M31). Surrounding us is a collection of dwarf galaxies and a "Council of Giants"—a ring of substantial galaxies including M81 and Centaurus A—encircling our position at a radius of about 12 million light-years (3.75 megaparsecs).¹

According to the standard model of cosmology, known as Lambda-CDM (Cold Dark Matter), gravity drives the formation of structure. Over billions of years, gravity should pull these galaxies together, creating large "peculiar velocities"—motions that deviate from the smooth, uniform expansion of the universe (the Hubble Flow). In a region containing as much mass as the Local Group, simulations typically predict a "hot" environment, where galaxies buzz around with high random velocities, falling in toward the center.²

However, observational reality has long contradicted this theoretical expectation. The "local Hubble flow"—the expansion of the universe as measured by galaxies within 3 to 10 million light-years—is strikingly "cold." The velocity dispersion, a measure of the random agitation of galaxies, is remarkably low, often measured below 30 to 50 kilometers per second.² This region is so calm that galaxies appear to be drifting apart almost exactly as the Hubble Law predicts, with minimal disturbance from local gravity.

This created a "mass discrepancy." To explain the motion of the Milky Way and Andromeda (which are falling toward each other), the Local Group must contain a vast amount of dark matter—over a trillion solar masses.⁵ Yet, if that mass existed in a simple sphere, it should be dragging nearby galaxies inward, breaking the "quiet" flow. For decades, astronomers struggled to explain how the Local Group could be so massive and yet so dynamically quiet.⁶

The resolution to this paradox arrived in January 2026. A team led by Ewoud Wempe of the Kapteyn Astronomical Institute, utilizing the Bayesian Origin Reconstruction from Galaxies (BORG) method, demonstrated that the mass is not missing; it is simply shaped differently than we thought. We are living on a vast, flat sheet of dark matter.²

2. The Architecture of the Local Sheet

The newly identified structure is a colossal wall of invisible mass that defines the geography of our local universe. It is not merely a statistical overdensity but a distinct physical object with defined boundaries and dynamics.

2.1 Dimensions and Geometry

The "Dark Matter Sheet" is a flattened structure extending tens of millions of light-years.

• Diameter: The sheet spans a diameter of approximately 10.4 megaparsecs (roughly 34 million light-years).⁹ This immense breadth encompasses the entire Local Group and the surrounding Council of Giants.

• Thickness: The vertical structure of the sheet is the subject of precise measurement. While the visible distribution of galaxies suggests a "thin" layer with a minor axis of about 0.465 megaparsecs (1.5 million light-years) ⁹, the underlying dark matter sheet is estimated to have a thickness of about 1.6 megaparsecs (5.2 million light-years).¹⁰ Even at this thicker estimate, the structure is dynamically flattened, resembling a pancake or a wall rather than a sphere.

• Density: The density of matter within the sheet is approximately twice the average density of the universe.¹⁰ Crucially, the study found that within a radius of 4 megaparsecs, the sheet contains over four times more mass than would be inferred from a spherical model fitting the same velocity data.²

2.2 The Void and the Wall

The sheet does not exist in isolation; it forms the boundary of the Local Void, a vast region of emptiness extending roughly 150 million light-years (45 megaparsecs).¹¹ Voids in the cosmic web act as "pushers." Because they contain so little mass, they exert very little gravity compared to the denser regions around them. The result is a net force directed away from the void.

Astronomer Brent Tully and colleagues have calculated that the Milky Way and the entire Local Sheet are moving away from the Local Void at approximately 260 kilometers per second.¹² This "expansion velocity" of the void compresses the matter at its edge, sharpening the Local Sheet into a distinct wall. The sheet is effectively the "crust" of the void, sweeping up matter as the void expands.¹²

Table 1: Key Parameters of the Local Dark Matter Sheet

Parameter	Value	Description
Diameter	~10.4 Mpc (34 Mly)	Extent of the flattened mass distribution 9
Thickness (Dark Matter)	~1.6 Mpc (5.2 Mly)	Vertical height of the mass concentration 10
Thickness (Galaxies)	~0.465 Mpc (1.5 Mly)	Vertical dispersion of visible galaxies 9
Velocity Dispersion	< 30 - 50 km/s	"Cold" characteristic of the in-plane flow 2
Void Repulsion	~260 km/s	Velocity away from the Local Void center 9
Mass Concentration	> 4x Spherical Model	Excess mass hidden in the sheet geometry 2

3. The Dynamics of "Floating": Solving the Cold Flow

The primary triumph of the Wempe et al. study is the resolution of the "Cold Hubble Flow" paradox. How can a region containing four times the expected mass appear so quiet? The answer lies in the geometry of gravity.

In a spherical model, gravity acts as a point source. A galaxy located a few million light-years away from the Local Group feels a strong pull toward the center. This pull acts as a brake on the cosmic expansion, slowing the galaxy's recession. If the mass is high, the braking is severe, causing high "peculiar velocities" (deviations from the smooth expansion).

In a sheet model, gravity behaves differently. The mass is distributed laterally across a vast plane. A galaxy within this plane feels the gravitational pull of the Milky Way and Andromeda at the center, but it also feels the gravitational pull of the rest of the sheet extending outward to 10 megaparsecs.

• The Tug-of-War: The mass in the outer regions of the sheet exerts an outward force on galaxies within the sheet.²

• Damping Effect: This outward pull counteracts the inward pull of the central Local Group. The net force on the galaxy is significantly reduced compared to the spherical case.

Consequently, galaxies in the Local Sheet "float" in a gravitational equilibrium. They are not falling violently toward the center because the floor they stand on (the sheet) is pulling them outward. This damping effect suppresses peculiar velocities, allowing galaxies to drift apart smoothly with the Hubble Flow.² The "coldness" is not due to a lack of mass, but due to the balanced forces of the sheet geometry.

4. Methodology: Reconstructing the Invisible

The discovery of this structure required a departure from traditional simulation techniques. Standard cosmological simulations (like the famous Millennium Run) evolve random patches of the universe. While they show that sheets and filaments exist, they cannot tell us if we live in one. To answer that, researchers need "constrained simulations"—models that are forced to match our specific local geography.

4.1 The BORG Algorithm

Wempe’s team employed the Bayesian Origin Reconstruction from Galaxies (BORG) algorithm.² This sophisticated method is an "inverse" problem solver. Instead of starting with the Big Bang and hoping to get the Milky Way, BORG starts with the observed positions and velocities of local galaxies and works backward in time.

1. Input: The 3D coordinates and motions of galaxies from catalogs like Cosmicflows-3.

2. Inference: The algorithm uses Bayesian statistics to explore millions of possible "initial conditions" of the early universe.

3. Evolution: It evolves these conditions forward using gravity solvers to see which ones produce a local universe that matches our observations.

By generating "virtual twins" of the Local Group, the researchers could look at the underlying dark matter distribution that accompanied the visible galaxies.⁶ They found that the models which best reproduced the "quiet" Hubble flow were invariably those where the Local Group was embedded in a massive sheet, rather than a filament or a spherical knot.⁷

4.2 Calibrating with Giants

The study also utilized data from massive, high-resolution simulations like IllustrisTNG and Uchuu.⁷ These provided the physical rules for how galaxies inhabit dark matter halos. By comparing the BORG reconstructions with these standard models, the team confirmed that the sheet-like geometry is a natural and expected outcome of the standard Lambda-CDM cosmology, provided one looks at the right environment.

5. The Broader Web and Implications

The identification of the Local Dark Matter Sheet has ripple effects that extend to our understanding of the broader cosmic web and the history of the Local Group.

5.1 Alignment with the Supergalactic Plane

The Local Sheet is not randomly oriented. It aligns closely with the Supergalactic Plane, a massive structure of galaxy clusters identified in the 1950s that includes the Virgo Cluster and the Great Attractor.² This alignment suggests that the Supergalactic Plane is not just a chance arrangement of bright galaxies but a fundamental "super-structure" of dark matter. The Milky Way’s local sheet is likely a small extension or a "sub-pancake" of this larger cosmic wall.¹⁷

This alignment explains the anisotropy (directionality) of local motions. Velocities are "cold" (quiet) within the plane of the sheet, but "hot" (fast) perpendicular to it. Galaxies above and below the sheet are raining down onto it, driven by the vertical gravity of the wall, while those within the sheet drift apart calmly.²

5.2 Resolution of the Timing Argument

One of the oldest problems in local cosmology is the "Timing Argument," first proposed by Kahn and Woltjer in 1959. They calculated that the Milky Way and Andromeda must be very massive (over 10^12 solar masses) to have reversed their expansion and begun falling toward each other.⁵

The sheet model preserves this high mass estimate while solving the problem of why this mass doesn't disturb the neighbors. The "extra" mass required by the Timing Argument is present—it is stored in the extended sheet. The sheet provides the binding energy to hold the Local Group together against the expansion, but its flattened geometry hides its gravitational influence from the more distant galaxies in the Hubble flow.²

5.3 Echoes from the Early Universe

The formation of such a sheet is consistent with the "Zel'dovich Approximation," which predicts that matter in the universe collapses first into sheets (pancakes), then into filaments, and finally into clusters.¹⁹ The presence of a distinct Local Sheet suggests our region is dynamically "young"—it has collapsed into a wall but has not yet rolled up into a dense filament or cluster.

Observations of the distant universe using the Atacama Large Millimeter/submillimeter Array (ALMA) support this timeline. Massive galaxies like SPT0311-58 have been observed forming in dense, sheet-like environments when the universe was less than a billion years old.² These ancient structures may be the high-redshift progenitors of systems like our Local Sheet, confirming that "sheet-first" formation is a universal process.

6. Conclusion

The discovery that the Milky Way floats on a giant Dark Matter Sheet is a paradigm shift in near-field cosmology. It transforms our understanding of the Local Group from a simple binary system of galaxies into a complex, structurally rich environment defined by the geometry of the cosmic web.

This "hidden floor" beneath our galaxy explains the eerie quietness of our local cosmos. It reconciles the massive gravity of the Milky Way and Andromeda with the smooth flow of their neighbors, solving a paradox that has persisted for decades. By combining precision observations with advanced Bayesian simulations, astronomers have finally mapped the invisible architecture of our galactic home, revealing that we are drifting on a vast, dark continent of matter, suspended between the crushing gravity of the giants and the expanding emptiness of the void.

References and Data Sources

The information in this report is synthesized from the following research snippets:

• Primary Study: Wempe et al. (2026), Nature Astronomy.²

• Simulation Methods: BORG algorithm and IllustrisTNG/Uchuu data.⁷

• Local Void & Sheet Dynamics: Tully et al. regarding void expansion and sheet definition.⁹

• Structural Dimensions: Data on the 10.4 Mpc diameter and thickness variations.⁹

• Historical Context: Kahn and Woltjer (1959) Timing Argument.⁵

• Broader Context: Supergalactic Plane alignment and ALMA observations of progenitors.²

Works cited

1. arXiv:1410.1192v2 [astro-ph.CO] 16 Oct 2014, accessed February 2, 2026, https://arxiv.org/pdf/1410.1192

2. Scientists Discover the Milky Way Is Floating on a Vast Sheet of Dark Matter Stretching Millions of Light-Years - The Daily Galaxy, accessed February 2, 2026, https://dailygalaxy.com/2026/02/milky-way-floating-on-giant-dark-matter-sheet-astronomers-say/

3. Locally cold flows from large-scale structure | Monthly Notices of the Royal Astronomical Society - Oxford Academic, accessed February 2, 2026, https://academic.oup.com/mnrasl/article/415/1/L16/965166

4. Hubble flow around the Local Group | Monthly Notices of the Royal Astronomical Society | Oxford Academic, accessed February 2, 2026, https://academic.oup.com/mnras/article/393/4/1265/1005948

5. arXiv:astro-ph/9810069v1 5 Oct 1998, accessed February 2, 2026, https://arxiv.org/pdf/astro-ph/9810069

6. Flat dark matter sheet solves local galaxy motion puzzle - Space Daily, accessed February 2, 2026, https://www.spacedaily.com/reports/Flat_dark_matter_sheet_solves_local_galaxy_motion_puzzle_999.html

7. The mass distribution in and around the Local Group - ResearchGate, accessed February 2, 2026, https://www.researchgate.net/publication/400083578_The_mass_distribution_in_and_around_the_Local_Group

8. An old puzzle solved: astronomers discover the world is flat - Stockholms universitet, accessed February 2, 2026, https://utbildning.su.se/english/news/articles/2026-01-27-an-old-puzzle-solved-astronomers-discover-the-world-is-flat

9. Local Sheet - Wikipedia, accessed February 2, 2026, https://en.wikipedia.org/wiki/Local_Sheet

10. Goodbye to the idea of empty space around the Milky Way—a study reveals that we live in a sheet of dark matter, accessed February 2, 2026, https://eladelantado.com/en/milky-way-space-universe-study/

11. Local Void - Wikipedia, accessed February 2, 2026, https://en.wikipedia.org/wiki/Local_Void

12. Galaxy Clustering - Astronomy in Hawaii, accessed February 2, 2026, https://www2.ifa.hawaii.edu/research/Galaxy_Clustering.shtml

13. There's a Huge Void Near Our Galaxy. Its Mysterious Depths Have Just Been Measured, accessed February 2, 2026, https://www.sciencealert.com/meet-our-neighbouring-local-void-its-mysterious-depths-hold-dark-matter-secrets

14. Our Entire Galaxy Appears to Be Embedded in a Colossal Sheet of Dark Matter - Futurism, accessed February 2, 2026, https://futurism.com/space/entire-galaxy-sheet-dark-matter

15. [2406.02228] Constrained cosmological simulations of the Local Group using Bayesian hierarchical field-level inference - arXiv, accessed February 2, 2026, https://arxiv.org/abs/2406.02228

16. Milky Way is too big for its "cosmological wall" - Tech Explorist, accessed February 2, 2026, https://www.techexplorist.com/milky-way-too-big-cosmological-wall/56359/

17. Flat patterns in cosmic structure | Monthly Notices of the Royal Astronomical Society | Oxford Academic, accessed February 2, 2026, https://academic.oup.com/mnras/article/526/3/4490/7296159

18. Slipher and the Nature of the Nebulae - Semantic Scholar, accessed February 2, 2026, https://pdfs.semanticscholar.org/3137/254e59c28950090d3d59be15779a607186be.pdf

19. The high-resolution density fields in a region of 100 h −1 Mpc base... - ResearchGate, accessed February 2, 2026, https://www.researchgate.net/figure/The-high-resolution-density-fields-in-a-region-of-100-h-1-Mpc-base-length-and-02-h-1_fig2_51961504

20. Constrained cosmological simulations of the Local Group using, accessed February 2, 2026, https://research.rug.nl/en/publications/constrained-cosmological-simulations-of-the-local-group-using-bay

21. Mapping the local cosmic web | Eberly College of Science, accessed February 2, 2026, https://science.psu.edu/news/Jeong5-2021

22. formation of Local Group planes of galaxies | Monthly Notices of the Royal Astronomical Society | Oxford Academic, accessed February 2, 2026, https://academic.oup.com/mnras/article/436/3/2096/1244842