Since we will not be able to discover dark matter particles, it is useful to consider alternatives. Although dark matter is the most supported model, the alternatives fall into two broad paths. The first is that we need to turn to extended models of general relativity, such as coherent gravity. Another argues that we need to change the nature of Newtonian dynamics. The first approach tends to be popular with theorists because it focuses on abstract theory in the same vein as Einstein’s original ideas. The latter, commonly known as modified Newtonian dynamics or MoND, tends to be more popular with observational astronomers.

MoND has a long history in astronomy. It was first introduced by Mordechai Milgrom in the early 1980s as a way to explain the rotation curves of galaxies. In our solar system, distant planets rotate more slowly than planets around the Sun; this is in perfect agreement with Newton’s law of universal gravitation. However, stars in galaxies revolve around the galactic center at roughly the same speed, regardless of distance. The explanation for dark matter is that most of the galaxy’s mass is scattered around the galaxy as a halo, we can’t see it because dark matter doesn’t interact with light. MoND claims that Newton’s famous equation F = ma is an approximation that only works for larger forces. For extremely small forces, such as the galactic drift of a star, the acceleration deviates only enough to match the galactic rotation curve.

If that sounds like a chore, it is. It breaks the fundamental laws of physics, doesn’t fit well with special relativity, and has many other problems. But it’s important to keep in mind that this was originally proposed as a proof-of-concept, not a solid theory. So that was the case with dark matter in the early years.

One of the reasons for MoND’s lack of popularity is that it is not a field theory. Electromagnetism, relativity theory, quantum theory, and others treat physical quantities as fields that allow physicists to apply all kinds of modern mathematical analysis. As MoND is not a field theory it lacks the global structure that puts it on an even playing field, forgive the pun. But there is a variant of MoND known as AQUAL which represents Quadratic Lagrangian. AQUAL’s predictions differ slightly from MoND, but AQUAL is a field theory and therefore popular with dark matter proponents. AQUAL also has a relative extension known as Tensor-Vector-Scalar Gravity (TeVeS).

While not popular in the scientific community, AQUAL is a valid testable model. Most of his predictions are also made by dark matter models, but there are a few that differ. The first is that there must be a twisting of the galaxy’s spin curve as the MoND effect becomes more dominant, and by 2022 this has been observed in more than 100 galaxies. Now, a study on binary stars seems to support AQUAL.

This study looked at large binary systems rather than galaxies. These are binary stars that rotate against each other at great distances. Because the gravitational attraction between the stars is small, Newtonian gravity and AQUAL make slightly different predictions about the motion of the stars. And since dark matter doesn’t have a strong influence on the scale of stellar systems, he speculates that stars should follow Newtonian motion.

Using data from the Gaia space telescope, the author analyzed the orbital motion of 26,500 large binary systems and compared the gravitational accelerations of the stars with their orbital separations. At higher accelerations the orbits are consistent with both AQUAL and Newtonian gravity, but at lower accelerations the double orbit deviates from the Newton just as AQUAL predicted. In other words, for thousands of binary systems, AQUAL is a better-than-Newtonian model of their motion. This seems to contradict dark matter models.

This is just a study and the author tries not to use bold statements. But this is a very interesting result. Dark matter remains the better theory for things like galaxy clusters and gravitational lensing effects, but it seems clear that AQUAL is a contender that can’t be easily ignored. Source