Using Newton’s physical rules, we can precisely simulate the motions of Solar System planets. In the early 1970s, scientists realized that this wasn’t the case, since disc galaxies’ outermost stars, far from the gravitational influence of all the mass at their center, were moving considerably faster than Newton’s theory anticipated.

Therefore, physicists argued that an invisible element known as “dark matter” was supplying extra gravitational attraction, causing the stars to accelerate – a generally accepted idea. In a new review, however, my colleagues and I argue that facts spanning a large variety of scales are better explained by an alternative explanation of gravity known as Milgromian dynamics or Mond, which does not require the existence of unseen matter. In 1982, Israeli scientist Mordehai Milgrom initially proposed it.

Mond’s basic hypothesis is that when gravity becomes extremely weak, as it does near the outskirts of galaxies, it begins to behave differently than Newtonian physics predicts. More than 150 galaxies have been found to have stars, planets, and gas in their peripheries that rotate faster than predicted by their visible mass alone. This phenomenon can be explained in this way. Nevertheless, Mond does not only explain these rotation curves; in many instances, it also predicts them.

According to philosophers of science, this ability to forecast makes Mond superior than the mainstream cosmological model, which claims that there is more dark matter than visible matter in the universe. This is due to the fact that, if this hypothesis is correct, the quantity of dark matter in galaxies is largely undetermined and is reliant on the galaxy’s origin, which is not always known. This makes it impossible to forecast the rotational velocity of galaxies. However, such predictions are often made by Mond and have thus far been accurate.

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Imagine that we know the apparent mass distribution of a galaxy but do not yet know its rotation speed. In the traditional cosmological model, it is only possible to predict with reasonable certainty that the rotation speed will be between 100 and 300 kilometers per second on the planet’s periphery. Mond provides a more precise forecast that the rotation speed must be between 180 and 190 kilometers per hour.

If future observations reveal a rotation speed of 188 km/s, this would be consistent with both ideas, although Mond’s theory is definitely preferable. This is a modernized version of Occam’s razor, which states that the simplest solution is preferable to more complex ones, in this instance that observations should be explained with the fewest “free parameters” feasible. Free parameters are constants that must be plugged into equations in order for them to function. However, their values are not predetermined by the theory; there is no compelling reason for them to take on any particular value. Newton’s constant of gravitation, G, and the mass of dark matter in galaxies predicted by the standard cosmological model are two such instances.

The term “theoretical flexibility” was created by us to summarize Occam’s razor, which asserts that a theory with more free parameters is consistent with a wider variety of evidence, but is also more difficult to understand. We tested the traditional cosmological model and Mond against numerous astronomical observations, including as the spinning of galaxies and the motions inside galaxy clusters, using this idea in our review.

Every time, we assigned a theoretical flexibility score ranging from -2 to +2. A score of -2 implies that a model’s prediction is clear and accurate without data inspection. In contrast, +2 suggests “anything goes” – theorists might have accommodated nearly any conceivable observable outcome (because there are so many free parameters). We also graded the degree to which each model corresponds to the observations, with a score of +2 signifying outstanding agreement and a score of –2 reserved for observations that blatantly contradict the theory. We next remove the theoretical flexibility score from the agreement with observations score, as a good fit with the data is preferable than one that can fit anything.

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A excellent theory would offer precise predictions that are later validated, ideally achieving a total score of +4 in a variety of tests (+2 -(-2) = +4). A poor idea would receive a score between 0 and -4 (-2 minus (+2) equals -4). In this situation, precise predictions would fail, as they are unlikely to function with the incorrect physics. The standard cosmological model scored -0.25 on 32 tests, whereas Mond got +1.69 on 29. The scores for the standard cosmological model and Mond on a variety of tests are depicted in Figures 1 and 2, respectively.

The difficulties with dark matter

Spiral galaxies often feature “galaxy bars” in their centers, which are rod-shaped luminous patches consisting of stars, and this is one of the most egregious problems with the standard cosmological model (see lead image). The bars revolve gradually. If galaxies were surrounded by huge halos of dark matter, their bars would move more slowly. Nevertheless, the majority, if not all, observed galaxy bars are swift. This disproves the mainstream cosmological model with a high degree of certainty.

The first models that claimed galaxies have dark matter halos made a significant error when they thought that the dark matter particles contributed gravity to the surrounding matter but were unaffected by the gravitational pull of regular matter. This calculation simplification does not reflect reality. When this was accounted for in future simulations, it became evident that dark matter halos surrounding galaxies cannot adequately explain their properties.

We studied numerous more flaws of the mainstream cosmological model, with Mond frequently able to naturally explain the findings. Popularity of the mainstream cosmological model may be due to computational errors or a lack of information regarding its flaws, some of which were only recently uncovered. It may also be a result of people’s reluctance to modify a gravity theory that has been so successful in other fields of physics.

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The significant advantage of Mond over the traditional cosmological model in our investigation led us to the conclusion that Mond is highly supported by the available data. We do not claim that Mond’s theory is perfect, but we do believe that it gets the big picture right: galaxies truly lack dark matter.

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