It might be time to ditch dark matter in favor of a new theory of gravity based on what we’ve found in our review

We can model the motions of planets in the Solar System quite accurately using Newton’s laws of physics. But in the early 1970s, scientists noticed that this didn’t work for disk galaxies—stars at their outer edges, far from the gravitational force of all the matter at their center—were moving much faster than Newton’s theory predicted.

This made physicists propose that an invisible substance called “dark matter” was providing extra gravitational pull, causing the stars to speed up—a theory that’s become hugely popular. However, in a recent review, my colleagues and I suggest that observations across a vast range of scales are much better explained in an alternative theory of gravity proposed by Israeli physicist Mordehai Milgrom in 1982 called Milgromian dynamics or Mond—requiring no invisible matter.

Mond’s main postulate is that when gravity becomes very weak, as occurs at the edge of galaxies, it starts behaving differently from Newtonian physics. In this way, it is possible to explain why stars, planets, and gas in the outskirts of over 150 galaxies rotate faster than expected based on just their visible mass. But Mond doesn’t merely explain such rotation curves, in many cases, it predicts them.

Philosophers of science have argued that this power of prediction makes Mond superior to the standard cosmological model, which proposes there is more dark matter in the universe than visible matter. This is because, according to this model, galaxies have a highly uncertain amount of dark matter that depends on details of how the galaxy formed—which we don’t always know. This makes it impossible to predict how quickly galaxies should rotate. But such predictions are routinely made with Mond, and so far these have been confirmed.

Dark matter: our review suggests it's time to ditch it in favour of a new theory of gravity

Comparison of the standard cosmological model with observations based on how well the data matches the theory (improving bottom to top) and how much flexibility it had in the fit (rising left to right). The hollow circle is not counted in our assessment, as that data was used to set free parameters. Reproduced from table 3 of our review

The problems with dark matter

One of the most striking failures of the standard cosmological model relates to “galaxy bars”—rod-shaped bright regions made of stars—that spiral galaxies often have in their central regions (see lead image). The bars rotate over time. If galaxies were embedded in massive halos of dark matter, their bars would slow down. However, most, if not all, observed galaxy bars are fast. This falsifies the standard cosmological model with very high confidence.

Another problem is that the original models that suggested galaxies have dark matter halos made a big mistake—they assumed that the dark matter particles provided gravity to the matter around it, but were not affected by the gravitational pull of the normal matter. This simplified the calculations, but it doesn’t reflect reality. When this was taken into account in subsequent simulations it was clear that dark matter halos around galaxies do not reliably explain their properties.

Dark matter: our review suggests it's time to ditch it in favour of a new theory of gravity

The barred spiral galaxy UGC 12158.

There are many other failures of the standard cosmological model that we investigated in our review, with Mond often able to naturally explain the observations. The reason the standard cosmological model is nevertheless so popular could be down to computational mistakes or limited knowledge about its failures, some of which were discovered quite recently. It could also be due to people’s reluctance to tweak a gravity theory that has been so successful in many other areas of physics.

The huge lead of Mond over the standard cosmological model in our study led us to conclude that Mond is strongly favored by the available observations. While we do not claim that Mond is perfect, we still think it gets the big picture correct—galaxies really do lack dark matter.

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