According to our current theories of physics, all the matter-energy in the universe consists of about 68.5% dark energy, 26.5% dark matter, and about 5% regular matter. The dark matter is believed to be around galaxies in a halo while dark energy is everywhere. The problem is that we have not so far been able to directly detect any dark matter particles despite strenuous and expensive efforts. After each failure to detect a signal, the debate is always whether to give up the search and declare that dark matter does not exist or to build a bigger, more sensitive detector with other materials in the hope that it will work. As I discuss in my book The Great Paradox of Science, this is a recurring situation in the history science. At any given time, in addition to the dominant paradigm, there are always other competing paradigms seeking to dethrone the champion. The fortunes of the competitors depend upon he fortunes of the dominant one and in the case of dark matter, competitors see an opening in the failure to detect it.
Dark matter has, in addition to explaining the velocities of stars in the arms of spiral galaxies that originally triggered the idea, also has some supporting evidence such as some Cosmic Microwave Background (CMB) data.
There are always many possible alternative theories that can be proposed. In the case of dark matter, other particles have been proposed to the dominant idea that dark matter consists of what are known as WIMPs (Weakly Interactive Massive Particles).
Then there are those theories that say that dark matter does not exist at all and that what is needed is a modified theory of gravity. The most prominent of these is known as MOND (Modified Newtonian Dynamics). It is not easy to dethrone a dominant paradigm. As I discuss in my book, it requires that the competitor be able to explain key features that the dominant theory explains. In addition, it should make a prediction that can be tested.
In the case of MOND they have not made much headway in explaining some of the features that dark matter purports to explain. Like all new theories, it takes time to develop and it is still early days. But it has been able to gain some adherents and this month saw the appearance of a paper that proposed a relativistic form of MOND that the authors say said can explain some features of the CMB.
This month however, researchers Constantinos Skordis and Tom Zlosnik from the Czech Academy of Sciences published a paper in the journal Physical Review Letters suggesting that a new modification to the parameters of Newton’s theory of gravity could provide an answer as to why dark matter has yet to be detected. And unlike previously proposed MOND theories, this one just might stick because the new proposal can match observations of the cosmic microwave background (CMB), which is a key detail that has lacked in the previous MOND-like theories.
“This new paper suggests a way of having what is called a relativistic extension of MOND, a theory that is more general that can be applied to the universe at large,” Avi Loeb, the former chair of astronomy at Harvard University who was not involved in the paper, told Salon. “It introduces some new fields, and according to the authors, seems to satisfy both what happens on the scale of galaxies, as well as what happens on the scale of the universe and that’s quite a feat.”
You can read the paper here (paywall).