A Dark DimensionIf dark energy and dark matter do interact, it could mean that they have a common origin, Khoury said.
This led them to propose in 2022 that dark matter and dark energy could share a link with a so-called dark dimension.
Changes in the size of the dark dimension would affect both dark energy and dark matter.
If dark matter has a stronger gravitational attraction to other dark matter than it does to ordinary matter, a special kind of “tidal tail” — an extended stream of stars, gas, and dust — would form behind one of the galaxies.
Understanding dark energy and dark matter and the relationship they may share is a daunting problem, he said.
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%.
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.
A Dark Dimension
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.
Cumrun Vafa, a physicist at Harvard University, thinks the mass of dark matter may vary over time. Hayward Photography
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.
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.
In a July 2025 paper, Obied and Vafa, along with Alek Bedroya of Princeton and David Wu of Harvard, found that the scenario proposed in 2019 was consistent with the DESI data. Their model predicts that the strength of dark energy and the mass of dark matter will decrease over time and that the rate at which dark energy changes will be proportional to its energy density. Because astrophysical measurements tell us that the energy density of dark energy is extraordinarily small, “it won’t change fast,” Vafa said.
“It’s not surprising that we didn’t see it until now,” he said, because the rate of change is so tiny. “We had to wait the entire age of the universe to detect something that small.”
If dark matter is coupled to dark energy, dark matter particles could interact with one another through a new, long-range force that is separate from gravity. The good news, Obied said, is that “there could be astrophysical ways of testing this.”
Coincidentally, two physicists — Marc Kamionkowski, now at Johns Hopkins University, and Michael Kesden, now at the University of Texas at Dallas — have already looked into one such test. In a paper published in 2006, they imagined a sequence of events in which two galaxies come close to each other and the gravity of one galaxy tugs at the other. If dark matter has a stronger gravitational attraction to other dark matter than it does to ordinary matter, a special kind of “tidal tail” — an extended stream of stars, gas, and dust — would form behind one of the galaxies.
Kesden and Kamionkowski looked for this effect and didn’t find it, which enabled them to set an upper bound on the possible strength of the extra attractive force. The upper bound was about 20 times bigger than the number Vafa’s team proposed, so the predicted value fell well within the observational bound. “It is interesting that we are now finding connections between that fairly abstract work and observational and experimental work,” Kamionkowski said.
The fact that a prediction based on calculations from string theory roughly agreed with the astrophysical evidence does not confirm the validity of these “stringy” models. But any correspondence between string theory and experiment is gratifying to Vafa, who has spent the past four decades trying to wrest the theory from the purely conceptual realm to the point of generating testable predictions.
Obied, one of Vafa’s former Ph.D. students, is driven by the same goal. Understanding dark energy and dark matter and the relationship they may share is a daunting problem, he said. “And I think it’s very beneficial to come at this in different ways” — be it from observational cosmology, from particle physics, or from string theory.
“This is how people should do science,” Obied said. “I mean, it’s the job of theoretical physicists to explore everything that’s possible, to get all the possibilities on the table. And eventually, the data will help us decide.”
