Understanding how stars orbit the center of the Milky Way allows us to directly test major theories about the structure of the Universe. In recent decades, galactic rotation curves have been interpreted as indirect evidence of an invisible mass, dark matter. An alternative, the MOND (MOdified Newtonian Dynamics) theory, proposes adjusting the laws of gravity to very low acceleration, without invoking this hypothetical matter. But the latest data from the Gaia satellite reveal a clear drop in the rotation speed of our galaxy at great distances, an unexpected behavior.
In a study published in Astronomy & Astrophysics, Even Coquery (École Centrale de Lyon) and Alain Blanchard (University of Toulouse, IRAP, CNRS) analyze these results. Their objective: to determine if this drop can still be explained without dark matter, or if it constitutes a decisive argument in favor of the standard cosmological model.
A rotation curve that contradicts expectations
For several decades, astronomers have used the rotation curves of galaxies to probe their mass. These curves measure the speed at which stars rotate around the galactic center, as a function of their distance. According to classical physics, this speed should decrease beyond a certain radius, because the density of visible matter decreases. However, in the majority of observed galaxies, these speeds remain surprisingly constant. This phenomenon is attributed to an invisible matter. The famous “dark matter”, constituting approximately 85% of the total mass.
However, the Milky Way, our own galaxy, is more difficult to study from the inside. So far, measurements have indicated a generally flat rotation, consistent with the presence of dark matter. But the latest data from the Gaia satellite, analyzed in particular by Jiao et al. (2023), highlighted an unexpected trend. There is a gradual decline in the rotation speed of stars beyond 15,000 light years from the galactic center, by about 3.5 kilometers per second for every additional 3,200 light years. This decline is not anecdotal. It is now confirmed by several independent teams cited in the new study.
This drop, over more than 30,000 light years from the center of the galaxy, calls into question the idea of a systematically flat curve and requires us to reconsider theoretical explanations. For Even Coquery and Alain Blanchard, this signal is sufficiently clear to test the two major competing hypotheses. Namely: dark matter, as described by the standard ΛCDM model, and the modification of gravity proposed by the MOND theory.
A dark matter model that fits observations
To understand what is happening in our galaxy, researchers have reconstructed its visible structure in detail. They took into account the three major known components of the Milky Way: a central bulge, a disk of stars and a disk of gas. Each has a well-defined shape and mass, measured in previous studies.
But even adding up all this visible matter is not enough to explain the high speeds observed in the outer regions of the galaxy. To bridge this gap, the most widely used cosmological model, called ΛCDM, assumes that a large amount of invisible matter – dark matter – surrounds the galaxy in the form of an immense spherical halo.
In their study, the researchers tested this model by adjusting the shape and density of this halo. Result: with realistic parameters, this model makes it possible to accurately reproduce the drop in speed observed by Gaia, particularly beyond 50,000 light years from the center.
This approach gives a total mass for the Milky Way of about 428 billion times that of the Sun. It remains compatible with other independent measurements. In other words, by adding dark matter around the galaxy, we can explain why the stars rotate more slowly at the periphery, without needing to modify the laws of gravity. This is what the other theory tested, MOND, fails to do.
The MOND theory defeated by the declining curve
Since the 1980s, some scientists have proposed another explanation for the movement of stars in galaxies. Rather than adding invisible matter, they suggest modifying the laws of gravity. This is the principle of MOND theory (for Modified Newtonian Dynamics), developed by the physicist Mordehai Milgrom.
According to MOND, when the acceleration becomes very small — as is the case in regions far from the centers of galaxies — gravity behaves differently from what classical physics predicts. This modification would help explain why, in most galaxies, rotation speeds remain constant at great distances.
But this time, Gaia's data poses a serious problem for MOND. The drop in speed observed in the Milky Way does not correspond to what this theory predicts. The researchers tested MOND using the normal characteristics of our galaxy: star mass, disk shape, gas distribution. Even by adjusting the main parameter of the theory, called a₀ (the threshold from which gravity would be modified), they do not obtain a curve that sticks to the observations.
The best result gives a value of a₀ much higher than that used for other galaxies. Furthermore, the gap between the model and the data remains significant. This shows that MOND fails to explain this slowdown without deviating from values considered universal.
Even adjusted, MOND requires extreme assumptions
After observing that the MOND theory failed to explain the drop in speed observed in the Milky Way, the researchers wanted to give it one last chance. To do this, they tested whether MOND could work provided it modified the known characteristics of our galaxy.
They used a statistical method called MCMC (Markov chain). It allows you to explore thousands of possible combinations of parameters. This time, they no longer fixed the mass of the stellar disk, nor that of the gas, nor even the size and thickness of the disk. They also left the value of the key parameter of MOND, a₀, free. The goal: to see if there is a set of conditions under which MOND could stick to Gaia data.
The results are striking. For MOND to succeed in reproducing the observed drop in speed, the Milky Way would have to be much more massive than we think. The mass of the star disk is expected to increase threefold, reaching more than 100 billion times that of the Sun. This is far above current estimates based on observations. Furthermore, the value of a₀ which would allow this result is extremely low, even close to zero.
Clearly, MOND can only adapt by radically changing the properties of the galaxy. And this to the point of no longer being credible. The researchers conclude that there is no realistic combination that allows MOND to explain the observed rotation, without contradicting other reliable data. This test, based solely on our own galaxy, is enough to seriously doubt the universal applicability of this theory.
Source: E. Coquery, A. Blanchard. “Cosmological implications of the Gaia Milky Way evolving rotation curve”. Astronomy & Astrophysics2025.
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