The original version of this story appeared in Quanta Magazine.
For those who see the world as a dark place, the universe seems to offer little solace. According to current estimates, approximately 70 percent of the stuff that makes up the cosmos consists of dark energy, an unknown force that pushes space to expand. And another 25 percent consists of dark matter, a mysterious material that holds galaxies together.
But semantically speaking, dark energy and dark matter are not so much “dark” as they are invisible. They do not emit, reflect, nor absorb light, and they have so far proven impossible to observe directly.
Generally, dark energy and dark matter are regarded as separate entities, their elusiveness their primary link. But recent astronomical observations have spurred scientists to look further into a less popular idea—that the two are in fact physically intertwined.
In 2024, the research team known as DESI, for Dark Energy Spectroscopic Instrument, found evidence that the strength of dark energy, sometimes referred to as the “cosmological constant,” had lost its presumed constancy. A 2025 study based on more than twice as much data also concluded that dark energy was changing over time.
These results indicate that after attaining a maximum value about 2 billion years ago, dark energy may have begun to weaken. But the results also suggested that in an earlier era, dark energy could have grown stronger, in seeming defiance of the law of energy conservation.
Researchers describe this as dark energy entering the “phantom regime.” They liken the situation to a ball rolling uphill. It’s certainly possible, but only if the ball is under the influence of something other than gravity.
A slice of a 3D map produced by the Dark Energy Spectroscopic Instrument team shows connected strands of galaxies forming the structure of our universe.
A growing number of theorists are looking into the possibility that what’s influencing dark energy are its ties to dark matter.
Even though scientists have assumed that dark energy and dark matter “don’t have anything to do with each other,” said Tim Tait, a particle physicist at the University of California, Irvine, “you can imagine a case where one influences the other. And it would not be surprising if [they] were manifestations of a kind of unified theory of the dark universe.”
The idea that dark energy and dark matter interact is not new. For example, Justin Khoury, a physicist at the University of Pennsylvania, investigated the prospect in 2005.
Khoury explained that in that study, he and two coauthors posed a hypothetical question: Could there be a form of dark energy whose energy density increased over time? He and his collaborators found that if dark energy and dark matter could affect one another, they could produce what looked like (but was not) phantom behavior. “It is the most natural, simplest way of achieving this,” Khoury said.
Two decades later came the DESI finding that dark energy might actually be changing over time. The result spurred Khoury and two Pennsylvania colleagues, Meng-Xiang Lin and Mark Trodden, to construct a model of dark interactions based on a dark-sector analogue of quantum chromodynamics, a cornerstone theory of particle physics. In the new model, both the energy density of dark energy and the mass of dark matter change in concert.
Another recent study imagines something similar. According to this model, which was published in January in the journal Physical Review D, dark matter could have transferred a small fraction of its energy to dark energy during a previous era of cosmic history. “Dark matter is the main brake on [the universe’s] expansion,” said one of the authors, Elsa Teixeira, a cosmologist at the University of Montpellier in France, so easing up on that brake would have caused the expansion of the universe to accelerate.
Elsa Teixeira, a cosmologist at the University of Montpellier in France, investigated a cosmological model in which dark energy and dark matter interact.
The appearance of phantom behavior is basically the result of bookkeeping, said David Andriot, a physicist at CNRS, the French National Center for Scientific Research. “Any change or evolution of the mass of dark matter has been put into the box of dark energy,” said Andriot, who presented his own coupled dark energy-dark matter model in May 2025.
Cumrun Vafa, a physicist at Harvard University, agreed. “The notion that you can compute dark energy independently of dark matter is wrong,” he said. “That assumption, often made by cosmologists and also followed by the DESI team, led to the physically unacceptable phantom behavior.”
The DESI findings are not the only reason to consider an association between dark energy and dark matter. Teixeira and colleagues’ recent study shows how allowing the two to interact can, at least in some scenarios, ameliorate one of the most persistent problems in cosmology: the Hubble tension.
At issue is the current expansion rate of the universe, known as the Hubble constant. Its value can be measured by using light from the early universe, which provides a view of how the cosmos was growing in its earliest days. It can also be measured by drawing upon more recent phenomena, such as exploding stars called supernovas, which reveal how the universe is growing now.
Studying the light from supernovas like this one—the flash of light in the bottom right of an image from the Hubble Space Telescope—can tell us about the expansion rate of the universe.
According to the standard model of cosmology, that expansion rate should be the same no matter how you measure it. But in recent years, scientists have found that the expansion rate of the early universe and that of the more recent universe vary by approximately 9 percent.
That discrepancy, or tension, “has provoked heated debates in the cosmology community about whether this difference could be due to systematic errors or whether it is a signal of new physics,” Teixeira and her co-authors wrote. Their model posits that in a world where dark energy and dark matter interact, what seemed to be a crisis caused by disparate expansion rates instead becomes something to be expected.
If dark energy and dark matter do interact, it could mean that they have a common origin, Khoury said.
Ongoing work in string theory—the idea that our universe, at its smallest, most fundamental level, is made up of vibrating strings—suggests a way these concepts might be connected. In 2019, building on the idea in string theory that dark energy is naturally varying, Vafa and two collaborators concluded that the mass of dark matter particles may also vary over time. This led them to propose in 2022 that dark matter and dark energy could share a link with a so-called dark dimension.
String theory posits the existence of six or seven extra dimensions in addition to our familiar three dimensions of space and one of time. All of these dimensions are thought to be as small as physically possible—close to the Planck scale (10-35 meters)—but the researchers proposed that the dark dimension could be significantly bigger than the others—on the order of a micron (10-6 meters).
Gravitons, theoretical particles that impart the force of gravity, could leak into this enlarged dark dimension. If they did, they would pick up mass and become what are called dark gravitons. These massive gravitons would reside in the dark dimension, but their gravitational effects could be felt in other dimensions, allowing them to fulfill the role that is normally ascribed to dark matter.
Cumrun Vafa, a physicist at Harvard University, thinks the mass of dark matter may vary over time.
In this scenario, “there is a very natural coupling between dark energy and dark matter,” said Georges Obied, a physicist at the University of Chicago. Changes in the size of the dark dimension would affect both dark energy and dark matter.
