Gaia’s Milky Way Rotation Curve Favours Dark Matter Over MOND

Using millions of highly accurate stellar-motion measurements from Gaia, recent research suggests that the Milky Way’s rotation favours unseen mass over altered laws of gravity.

Gaia’s new map shifts the balance

Rotation curves show the orbital speeds of stars at varying distances from a galaxy’s centre. For many years, observations of numerous galaxies found “flat” curves, meaning that speeds remain almost unchanged at large distances. Such flatness implies there is more mass than can be accounted for by starlight and gas alone. This additional mass is usually represented by an enormous, roughly spherical dark-matter halo.

Studying our own galaxy is more difficult because we observe it from inside. Older maps indicated an almost flat rotation profile, but recent Gaia data releases have made the picture clearer. Several research groups, including Jiao and colleagues in a 2023 study, now find that orbital speeds steadily decrease beyond about 15,000 light-years from the centre. The decline is approximately 3.5 kilometres per second for every extra 3,200 light-years, continuing over more than 30,000 light-years. Separate teams using different tracers and techniques have independently found the same effect.

Gaia reveals a clean, sustained fall in rotation speed in the outer Milky Way, replacing the old idea of a strictly flat curve.

The result is important because it provides a clear test between two competing explanations: a dark-matter halo under the standard framework, or modified gravity theories designed to explain galactic motions without invisible mass.

Why rotation curves matter

With classical gravity and visible matter alone, orbital speeds should decrease at large radii. The flat curves seen in many galaxies therefore required a new explanation. Two main approaches developed. One introduces a halo of non-luminous particles to supply extra mass. The other, MOND (Modified Newtonian Dynamics), changes gravity at extremely low accelerations using a constant generally denoted by a0. Flat rotation curves can often be accommodated by MOND with a nearly universal a0. A persistent decline is, however, more difficult for this model to reproduce.

The standard model with hidden mass reproduces the decline

Even Coquery and Alain Blanchard constructed a detailed Milky Way mass model incorporating a central bulge, a stellar disc and a gas disc, all with measured masses and shapes. On their own, these visible components cannot account for the outer galaxy. The researchers therefore included a dark-matter halo with realistic properties based on the standard cosmological model.

When the halo density profile is varied within accepted limits, the model matches Gaia’s declining rotation measurements, particularly beyond 50,000 light-years. Achieving this fit does not depend on unusual assumptions about the galaxy’s visible matter.

Their best-fitting model gives a total mass of around 4.28×10^11 solar masses. This is well within the ranges derived from satellite orbits, stellar streams and the motions of halo stars. Neither the halo’s size nor its concentration is exceptional. Instead, it provides a credible reservoir of additional mass that progressively influences the velocity field at greater distances.

A conventional halo fit delivers a Milky Way mass of about 428 billion Suns and naturally produces the observed decline in speed.

• The observed outer-disc gradient is about −3.5 km/s per kiloparsec.
• The Sun lies near 8.2 kiloparsecs from the centre, within the region where the decline begins.
• Contributions from the bulge, disc and gas remain close to independent estimates based on star counts and emission maps.
• Halo properties remain compatible with simulations and limits derived from satellite galaxies.

Modified gravity encounters difficulties

MOND has remained attractive because it can follow flat rotation curves using one low-acceleration scale. Gaia’s Milky Way result presents a more demanding test. With conventional values for the disc, gas and bulge, and using widely adopted MOND versions, the predicted orbital speeds fail to drop as the observations show. Under those conditions, the closest fit requires an a0 far higher than the values that describe other galaxies. The disagreement persists even when generous uncertainties are included.

Even flexibility brings major trade-offs

The researchers then carried out a fully flexible Markov chain Monte Carlo search. Stellar and gas masses were allowed to vary substantially, while the disc’s thickness and scale length were loosened and a0 was permitted to float. The aim was straightforward: identify any realistic set of parameters that could trace Gaia’s measured decline.

The analysis does produce a mathematical fit, but only with substantial costs. The stellar disc would have to be about three times heavier than standard estimates, exceeding 100 billion solar masses. That would contradict star counts, stellar-population models and independent dynamical measurements. Meanwhile, the a0 value that brings MOND nearest to the data becomes extremely small, approaching zero in some chains. This removes the very gravitational modification on which the theory is based.

In simple terms, MOND can only come close to Gaia’s trend by distorting fundamental Milky Way properties far beyond plausible limits, or by forcing its central parameter into a range that defeats its purpose.

Aspect	Dark-matter halo	Modified gravity (MOND)
Match to declining speeds	Reproduced with a realistic halo profile	Poor with standard parameters
Stellar disc mass needed	Close to literature values	~3× higher than observations
Key parameter behaviour	No special tuning	a0 moves into unrealistic ranges
Consistency with other data	Agrees with streams and satellites	Conflicts with independent constraints

What might still mislead us

Rotation curves may be affected by motions that are not fully circular. The central bar creates streaming motions, while spiral arms disturb both gas and stars. At large radii, the disc is warped and flared. Asymmetric drift also changes how stellar tracers behave relative to gas. Calibration further depends on the Sun’s precise distance from, and speed relative to, the galactic centre.

Recent investigations have accounted for these systematic effects. Different groups have applied distinct tracers and corrections, yet the declining pattern remains across approaches, increasing confidence in the finding. Even so, better treatment of non-circular motions and selection effects should reduce the uncertainties in future data releases.

Why this matters beyond theory labels

Dark-matter halos are more than a way of balancing the mass budget. Their forms determine how satellite galaxies fall towards the Milky Way and are disrupted. They influence predictions of substructure capable of lensing background stars and galaxies. They also help establish local targets for direct-detection experiments on Earth, which depend on the density and velocity distribution of particles close to the Sun.

A falling rotation curve offers clues about how halo density varies with radius. This improves estimates of the local dark-matter density, an essential input for detectors. It also changes predictions for the trajectories of long, narrow stellar streams including GD-1 and Palomar 5. Those streams can, in turn, probe the halo’s clumpiness and its history of growth.

What to watch next

• Gaia’s next release will provide longer observational baselines, increasing velocity accuracy for faint stars at great distances.
• New 21-cm surveys will produce cleaner maps of outer gas, helping to distinguish circular motion from streaming.
• Very-long-baseline maser observations will better anchor distances and velocities towards the inner regions.
• Forthcoming wide-area surveys will map additional stellar streams, placing tighter limits on halo mass and shape.

Helpful context and definitions

a0 is the acceleration scale at which MOND diverges from classical gravity. Its usual value is derived from fitting many spiral galaxies. If one a0 applies everywhere, it supports the notion of a universal change in the law of gravity. If individual systems need different a0 values, the theory becomes less predictive.

Halo-mass estimates differ because they depend on the tracers being weighed and the distance out to which they are measured. The 4.28×10^11 solar-mass value in this case is the mass enclosed by the region explored by Gaia’s rotation data and the assumptions of the model. Estimates including very distant satellites may give larger totals because they sample a greater portion of the halo.

Try this mental model

Imagine the Milky Way’s visible disc as the exposed tip of an iceberg. Close to the centre, luminous matter supplies much of the gravitational weight. Further out, its contribution becomes thinner. If rotation remains flat, extra mass must take over the load. If rotation declines gradually, the additional mass is still present but distributed in a manner that permits speeds to fall. Gaia indicates that the second picture describes our galaxy.

Students and enthusiasts can try a simple exercise: use a published rotation curve, subtract the calculated contributions of stars and gas, then assess the halo density profile needed to account for what remains. Varying the disc mass within observational limits shows how strongly the outer curve continues to require unseen mass.