Most nearby galaxies are receding — only Andromeda approaches, and we finally know why

Lead

Researchers report that the puzzling local pattern of galactic motions — with almost every massive neighbor receding while only Andromeda moves toward us — is best explained by a vast, flat sheet of mass dominated by dark matter. Using constrained cosmological simulations run from the early universe to the present, teams matched the positions and velocities of the Milky Way, Andromeda and 31 nearby galaxies out to roughly 32 million light-years and found the surrounding mass lies in a flattened plane. The configuration counteracts the Local Group’s inward pull on many neighbors, so most outside galaxies are dragged away faster than simple Hubble expansion would predict. The result reconciles local galaxy motions with the standard lambda cold dark matter model while isolating Andromeda as the lone major inbound galaxy.

Key Takeaways

• The Andromeda Galaxy (M31) lies about 2.5 million light-years away and is moving toward the Milky Way at roughly 68 miles per second (110 km/s).
• Simulations matched observed data for 31 galaxies outside the Local Group out to ~32 million light-years, reproducing their anomalous recession speeds.
• Researchers find the mass just beyond the Local Group is distributed in a tens-of-millions-of-light-years–wide flattened sheet of dark and visible matter.
• This flat mass sheet pulls neighboring galaxies outward, nearly cancelling the inward gravitational influence of the Milky Way and Andromeda beyond ~8 million light-years.
• Regions above and below the sheet are low-density Local Voids that expanded faster than average and funneled material into the sheet, explaining the lack of other inbound large galaxies.
• The models are consistent with the standard lambda cold dark matter (ΛCDM) cosmology and initial fluctuations seen in the cosmic microwave background.

Background

Since the mid-20th century astronomers have noted a tension between local galaxy motions and a simple Hubble expansion. In 1959 Franz Kahn and Lodewijk Woltjer argued that the Milky Way and Andromeda together must contain much more mass than the visible stars account for, implying extensive dark matter halos that could reverse local expansion. Later observations confirmed the Milky Way–Andromeda pair has the combined mass necessary to produce their mutual approach.

Nevertheless, most nearby large galaxies do not share Andromeda’s inward motion; they recede from the Milky Way, often at rates that exceed what uniform Hubble flow would predict. That pattern has been puzzling because, if mass were distributed spherically around the Local Group, external galaxies should be decelerated by the Group’s gravity and thus move more slowly away than Hubble’s law alone implies.

The new work addresses this mismatch by placing the Local Group within a more complex local environment: not an isotropic halo of external mass but a flattened, wall-like concentration of matter that alters gravitational tug locally and explains why only Andromeda is inbound.

Main Event

To probe the cause of the anomalous local velocities, researchers built constrained cosmological simulations that start from mass fluctuations encoded in the cosmic microwave background and evolve forward to today. The simulations were tuned to reproduce salient observed quantities: the masses, positions and velocities of the Milky Way and Andromeda, plus the positions and motions of 31 galaxies just outside the Local Group, reaching out to ~32 million light-years.

When the team compared outcomes to real-world measurements, the best-matching runs showed a prominent, flattened distribution of matter surrounding the Local Group. In projection this structure resembles a wide sheet or wall, tens of millions of light-years across, with the Milky Way and Andromeda embedded near its midline. Galaxies lying in the plane experience an outward pull from the larger sheet that offsets the Local Group’s inward gravity.

The simulations indicate a dividing scale near ~8 million light-years: galaxies closer than that move away from the Milky Way more slowly than Hubble flow predicts (i.e., somewhat bound), while those beyond it are receding faster than the Hubble expansion would imply. That spatial dependence arises because the sheet’s external mass partially cancels the Local Group’s decelerating influence on more distant neighbors, producing the observed velocity pattern.

Moreover, the regions above and below the sheet are underdense—the Local Voids—so there are simply few massive objects in those directions that could be falling toward us. The absence of other inbound giants is therefore not surprising; the mass that could produce inward motion was evacuated into the surrounding walls early in cosmic history.

Analysis & Implications

Layering a flattened external mass distribution onto the Local Group changes how we interpret local dynamics and the environment of the Milky Way. It shows that local departures from simple Hubble flow need not contradict ΛCDM if the three-dimensional mass topology is accounted for. In other words, global cosmological models and detailed local dynamics can be reconciled by allowing for anisotropic mass arrangements on tens-of-megaparsec scales.

For galaxy formation and interaction forecasts, the result narrows the set of plausible future encounters for the Milky Way. While a Milky Way–Andromeda merger remains likely on multi-gigayear timescales, the sheet geometry reduces the chance of many other massive neighbors being on collision courses. This alters long-term predictions for accretion, satellite capture and the growth of the Milky Way’s halo.

On observational fronts, the model predicts measurable flows: galaxies at higher latitudes relative to the sheet should be moving toward the plane, and those motions can reach a few hundred kilometers per hour in some regions. Confirming such infall signatures with independent distance and velocity surveys would strengthen the sheet hypothesis.

Finally, the finding has consequences for mapping the local cosmic web. If sheets like this are common, then interpreting surveys that assume roughly spherical mass distributions around galaxy groups risks biasing mass estimates and environmental classifications at low redshift.

Comparison & Data

The table summarizes the core numbers at play. The simulations use initial conditions constrained by the cosmic microwave background, then select realizations that reproduce present-day observables of the Local Group. The transition near ~8 million light-years is not a sharp physical boundary but an approximate scale where external sheet gravity overtakes the Local Group’s inward influence for measured tracers.

Reactions & Quotes

“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.”

Simon White — Max Planck Institute for Astrophysics (paraphrased)

Context: White emphasizes that a non-spherical external mass layout is required to reproduce the peculiar velocity pattern of local galaxies when the combined Milky Way–Andromeda mass is taken into account.

“We are exploring all possible local configurations of the early universe that ultimately could lead to the Local Group; it is great that we now have a model consistent with both cosmology and local dynamics.”

Ewoud Wempe — University of Groningen (paraphrased)

Context: Wempe notes the study’s constrained-simulation approach links early-universe conditions inferred from the cosmic microwave background to the present-day arrangement around the Local Group.

“Regions above and below the sheet are sparse; those Local Voids expanded faster and pushed material into the walls.”

Study team summary (paraphrased)

Context: This explains why most large nearby galaxies are not falling toward the Milky Way—there are simply few such objects in the directions that would produce inbound motion.

Unconfirmed

• The exact spatial extent of the flat mass sheet beyond the simulation boundary remains uncertain and requires deeper surveys to map directly.
• Reported infall of high-latitude galaxies into the sheet at several hundred km/h needs independent confirmation from additional distance and velocity measurements.
• How common similarly shaped sheets are around other galaxy groups is not yet established; more constrained reconstructions of other regions are required.

Bottom Line

Constrained cosmological simulations indicate the unusual local velocity pattern — Andromeda inbound while most other nearby massive galaxies recede — arises from the Milky Way sitting near a vast, flattened concentration of dark and visible matter. That sheet’s gravity offsets the Local Group’s inward pull on distant neighbors, aligning local motions with the expectations of ΛCDM once the true three-dimensional mass distribution is included.

For observers and modelers, the result highlights the importance of mapping the full geometry of local mass, not just spherical averages, when inferring masses, predicting mergers, or comparing local measurements to global cosmology. Future surveys targeting velocity flows and deeper reconstructions of the local cosmic web will test and refine this picture.

Sources

• Live Science — (science news report summarizing the study)
• Max Planck Institute for Astrophysics — (research institute / press information)
• University of Groningen — (academic institution; lead author affiliation and statements)