The dark matter label embodies our ignorance of the nature of most of the matter in the universe. It contributes five times more than ordinary matter to the cosmic mass budget. But we can’t see it. We infer its existence only indirectly through its gravitational influence on visible matter.
The standard model of cosmology successfully explains the gravitational growth of today’s galaxies and their cluster formation, which is caused by primordial fluctuations in an ocean of invisible particles with initially small random movements. But this “cold dark matter” could actually be a mixture of different particles. It could consist of weakly interacting massive particles; hypothetical particles such as axions; or even dark atoms that don’t interact with ordinary matter or light. We haven’t discovered any of these invisible particles yet, but we have measured the imprint of the fluctuations in their original spatial distribution as minor deviations in the brightness of the cosmic microwave background across the sky, the relic radiation left over from the hot Big Bang.
Many experiments look for signatures of different types of dark matter, both in the sky and in laboratory experiments, including the Large Hadron Collider. This search has so far been unsuccessful. In addition to certain types of elementary particles, primordial black holes have largely been ruled out as the dominant component of dark matter, with a limited open window in the area of the asteroid masses waiting to be eliminated.
In a work from 2005, I and Matias Zaldarriaga showed that cold particles of dark matter can gravitationally accumulate on scales up to the mass of the earth. Evidence of such tiny lumps of dark matter has not yet been found; Observers have only examined much larger systems, namely galaxies like our own Milky Way, which contain gas and stars as an inner core surrounded by a halo of dark matter.
As the seminal work by Vera Rubin shows, the dynamics of gas and stars in galaxies actually implies the existence of invisible mass in a halo that extends far beyond the inner area in which ordinary matter is concentrated. Surprisingly, in galaxies like the Milky Way, the need for dark matter occurs only in the outer region, where acceleration falls below a universal value roughly equal to the speed of light divided by the age of the universe. This is an unexpected fact within the standard dark matter interpretation. The basic taste of a universal acceleration threshold increases the possibility that we may not be lacking any invisible matter, but that the effect of gravity on the dynamics of visible matter changes at low accelerations.
This was the idea of Moti Milgrom, who in 1983 proposed a phenomenological theory of “modified Newtonian dynamics” (MOON) to explain the dark matter problem. Remarkably, his simple recipe for modified dynamics at low accelerations explains the almost flat rotation curves in many galaxy halos very well, even after four decades of investigation. As expected in MOON, all available data on galaxies the size of the Milky Way show a close correlation between the circular velocity at the edge of galaxies and the total amount of ordinary matter (also known as baryonic matter), which manifests the so-called “baryonic Tully” -fisherman relationship. “In a paper from 1995 I showed with my first PhD student, Daniel Eisenstein, that the narrowness of this relationship is not explained trivially in the standard interpretation of dark matter. Even if dark matter exists, MOON raises the fundamental question: Why do they lead Dark matter particles provide a fundamental acceleration scale in the dynamics of galaxies – is that an important clue to their nature?
MOON faces challenges on scales larger than galaxies. More massive systems such as galaxy clusters, in which Fritz Zwicky first postulated the existence of dark matter and coined its name, indicate a lack of mass, although its acceleration in MOON tends to be above the threshold scale. In addition, the acoustic vibrations detected with the greatest precision in the fluctuations in brightness of the cosmic microwave background imply the presence of a dominant component of matter that flows freely, in addition to ordinary matter and radiant fluids, which are closely coupled by electromagnetic interactions.
But what about the smallest scales? Together with my postdoc Mohammad Safarzadeh, I recently examined the latest data from the Gaia study of extremely faint dwarf galaxies, which are satellites of the Milky Way. We have shown that their behavior deviates from MOND’s expectations. Just like galaxy clusters, dwarf galaxies seem to argue against the universality of MOON on all scales.
Does MOON’s success on the Milky Way scales and its failures on both smaller and larger scales offer new insights into the nature of dark matter? One possibility is that dark matter is highly self-interacting and avoids galactic nuclei. In a 2011 article with Neal Weiner, I showed that a dark sector interaction similar to the electrical force between charged particles can facilitate the avoidance of galactic nuclei by dark matter, the effect at the high collision speeds characteristic of galaxy clusters decreases.
Another possibility that I suggested with Julian Muñoz in a 2018 article was inspired by the EDGES experiment, which reported an unexpected excessive cooling of hydrogen atoms during cosmic dawn. We have shown that when some dark matter particles have a low electrical charge they can scatter ordinary matter and cool hydrogen atoms below expectations, as reported.
The explanation of an anomaly by the assumption that some of the dark matter particles are slightly electrically charged is far more speculative than the explanation of six anomalies by the assumption that the interstellar object ‘Oumuamua is a thin film pushed down by sunlight . Still, speculations about the nature of dark matter receive far more federal funding and general legitimacy than searches for techno-signatures from alien civilizations.
More precise clues are needed to find out the nature of dark matter. We hope that the coming decades will bring a solution to this cosmic mystery where all the pieces of the puzzle fit together. Alternatively, we could find a smarter child on the cosmic pad who whispers the answer in our direction. Although it feels like cheating on an exam, we should keep in mind that there is no teacher in sight to look over our shoulders.
This is an opinion and analysis article.