'Dark force' theory could solve 2 open cosmic mysteries

An illustration of a bright galaxy surrounded by a blueish halo. The background of space takes up a large portion of the image.
An illustration of the dark matter halo that surrounds the Milky Way. (Image credit: ESO/L Calçada)

A new theory that suggests dark matter is made up of particles that strongly interact with each other via a so-called "dark force." If true, this may finally explain the extreme densities we see in dark matter haloes surrounding galaxies.

The existence of particles called self-interacting dark matter (SIDM) acts as an alternative to cold dark matter theories which suggest the elusive stuff is made up of massive, slow-moving (and thus cold), weakly interacting particles that don’t collide. The problem with those cold dark matter models is that they struggle to explain two puzzles surrounding what are known as dark matter haloes. 

"The first is a high-density dark matter halo in a massive elliptical galaxy. The halo was detected through observations of strong gravitational lensing, and its density is so high that it is extremely unlikely in the prevailing cold dark matter theory," Hai-Bo Yu, team leader and a professor of physics and astronomy at the University of California, Riverside, said in a statement. 

"The second," he continued, "is that dark matter halos of ultra-diffuse galaxies have extremely low densities, and they are difficult to explain by the cold dark matter theory."

Related: Dark matter may be hiding in the Large Hadron Collider’s particle jets

The haloes

Dark matter presents a major conundrum for scientists because, despite making up around 85% of the matter in the cosmos, it does not interact with light and therefore remains virtually invisible to us. This tells researchers that dark matter can't just be unseen conglomerations of matter made up of electrons, protons and neutrons  —  so-called baryonic matter that comprises stars, planets, our bodies and pretty much everything we see around us on a day-to-day basis. No, dark matter has to be made of something else.

The only way that researchers can infer the existence of dark matter at all, in fact, is because it has mass and thus interacts with gravity. This effect can be "felt" by baryonic matter we can indeed see and by light, which astronomers are definitely able to observe. 

More specifically, when light travels past these dark matter-wrapped galaxies from background sources, the substance's influence on the fabric of space diverts the light's path and, in turn, makes the background sources appear "shifted" to new locations in space. 

This effect, dubbed gravitational lensing, is what originally allowed scientists to determine that most, if not all, galaxies are surrounded by haloes of dark matter in the first place. And these haloes are believed to extend far beyond the limits of those galaxies' visible matter objects like stars, gas and dust. Gravitational lensing has also allowed astronomers to measure the density of dark matter haloes. Denser haloes are responsible for stronger lensing than less dense haloes around ultra-diffuse galaxies  —  low-brightness galaxies with scattered gas and stars. However, researchers have struggled to explain the extremes of dark matter halo densities.

Enter, artificial intelligence

To tackle this puzzle, Yu and colleagues, including the University of Southern California postdoctoral researchers Ethan Nadler and Daneng Yang, constructed high-resolution simulations of cosmic structures that are based on actual astronomical observations.

They factored into these simulations strong dark matter self-interactions on mass scales relating to strong lensing haloes and ultra-diffuse galaxies. 

"These self-interactions lead to heat transfer in the halo, which diversifies the halo density in the central regions of galaxies," Nadler explained. "In other words, some halos have higher central densities, and others have lower central densities, compared to their cold dark matter counterparts, with details depending on the cosmic evolution history and environment of individual halos."

The team concluded that SIDM interacting through a "dark force," just as baryonic particles interact through the force of electromagnetism and via the strong and weak nuclear forces, could offer a solution that cold dark matter theories don’t deliver.

"Cold dark matter is challenged to explain these puzzles. SIDM is arguably the compelling candidate to reconcile the two opposite extremes," Yang added. "Now there is an intriguing possibility that dark matter may be more complex and vibrant than we expected."

The team thinks their research also provides an example of the analytical power of uniting real observations of the universe, which grow in detail with each new generation of telescope, with the burgeoning power of artificial intelligence

"We hope our work encourages more studies in this promising research area," Yu said. "It will be a particularly timely development given the expected influx of data in the near future from astronomical observatories, including the James Webb Space Telescope and upcoming Rubin Observatory."

The team’s research was published in November in The Astrophysical Journal Letters. 

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Robert Lea
Contributing Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.

  • Questioner
    A 'dark force' would at least address the highly coherent patterns observed around galaxies.

    DM's almost like a big quasi-rigid structure that demonstrates/shows zero responses to any galactic gravity including its own.
    To my mind these configurations border on artificial in character.

    Then the question is what does the form of a galaxy hang on as it moves around?
    The central black hole or some supposedly 'natural' contrivance of DM?

    Since DM doesn't respond to gravity what other forces could whip it up into the forms it is inferred to occupy?
    Other new 'dark' physics forces?
    Galactic constructing super aliens?
    Can't be any forces that affect baryonic matter because we could see those in the baryonic matter behavior.

    In the Milky Way we see no local inequitable distributions of DM.
    It is so perfectly uniformly distributed that it is exactly equivalent to it not being there in the first place.
    That's one reason I think it is happening underneath/bypassing space-time. Not actually gravity at all. A direct relationship of spatial ambiguity with a galaxy's central black hole, which is what everything in a galaxy, including DM is centered on.

    DM is like herding cats concentrically with the actual visible galaxy.
  • Atlan0001
    With a multiverse universe extreme density in-depth of verses regarding the folds receding away of the multi-dimensional multiverse is at once extreme shallowness apparent on the ocean's superficial 2-dimensional asymptotic surface of the universe. A traveler wouldn't swim the observed, observable, 2-d surface. the traveler would swim the unobserved, unobservable, volume-energy depths of multi-dimensional multiverse 'surfs' (currents) in/under/over the observable surface.

    An old picture told of nature is that the beauty is observably surface deep and the ugly unobservable in-depth in/under the surface . . . or vice versa since beauty is in the eyes of the beholder.

    SPACE! is unobservable (just as I've claimed many, many times)! Observable coordinate relative SPACETIMEs (plural) from the instant of photo-frame emissions are cartoon-page past histories (past histories from the instant of emissions) . . . no more, no less. No matter how fast light travels (the 1-d points thick, the 2-d photo-frames thick, travel) spontaneous REALTIME has already passed up the recorded time, the history.