Challenging Perspectives on Dark Energy and Dark Matter
For those who perceive the universe as an inherently bleak realm, the insights of contemporary cosmology may seem scarcely comforting. Current estimates suggest that approximately 70% of the universe’s composition is attributed to dark energy, an enigmatic force believed to be driving the accelerated expansion of space. Meanwhile, another 25% consists of dark matter, a mysterious substance that plays a crucial role in holding galaxies together.
However, in a semantic sense, both dark energy and dark matter are not so much “dark” as they are invisible. They neither emit, reflect, nor absorb light, rendering direct observation of these phenomena impossible. Typically considered as distinct entities, the elusive nature of dark energy and dark matter has been their only commonality. Recent astronomical observations, however, have spurred scientists to reconsider a less conventional idea: that these two components may indeed be physically interconnected.
In 2024, the Dark Energy Spectroscopic Instrument (DESI) team uncovered evidence suggesting that the intensity of dark energy, sometimes referred to as the “cosmological constant,” has not remained constant as previously assumed. A subsequent study in 2025, which analysed more than double the data, further supported the notion that dark energy is evolving over time.
These findings imply that after reaching a peak around 2 billion years ago, dark energy may have begun to wane. Intriguingly, they also indicate that dark energy could have experienced a strengthening phase in the distant past, seemingly defying the law of conservation of energy. Researchers have termed this phenomenon as dark energy entering a “phantom regime,” likening it to a ball rolling uphill—feasible only if influenced by forces beyond gravity.
Exploring Interactions Between Dark Entities
An increasing number of theorists are investigating the possibility that the dynamics of dark energy may be influenced by its interactions with dark matter. Tim Tait, a particle physicist at the University of California, Irvine, suggests that while scientists have traditionally viewed dark energy and dark matter as unrelated, it is conceivable that one could affect the other. “It would not be surprising if both were manifestations of a unified theory of dark universe,” he remarks.
The concept of interaction between dark energy and dark matter is not entirely new. In 2005, physicist Justin Khoury from the University of Pennsylvania examined this potential link, posing a hypothetical question: could there exist a form of dark energy whose energy density increases over time? He and his collaborators discovered that if dark energy and dark matter influenced each other, they could exhibit behaviour that appeared—yet was not—phantom-like. “It’s the most natural and straightforward way to achieve this,” Khoury asserted.
Fast forward two decades, and DESI’s findings that dark energy may be changing over time have prompted Khoury, alongside colleagues Meng-Xiang Lin and Mark Trodden, to develop a model of dark interactions. This model draws an analogy to the dark sector of quantum chromodynamics, a fundamental theory in particle physics. In this framework, the energy density of dark energy and the mass of dark matter are interdependent.
Addressing the Hubble Tension
A recent model published in January in Physical Review D supports a similar notion, proposing that dark matter could have transferred a small fraction of its energy to dark energy during an earlier epoch of cosmic history. “Dark matter is the primary brake on the universe’s expansion,” noted co-author Elsa Teixeira, a cosmologist at the University of Montpellier in France. A reduction in this braking force would have consequently accelerated the universe’s expansion.
The emergence of phantom-like behaviours can be attributed to this accounting, as explained David Andriot, a physicist at the French National Centre for Scientific Research (CNRS). “Any change or evolution in the mass of dark matter has been classified under the umbrella of dark energy,” he elaborated, having introduced his own coupled model of dark energy and dark matter in May 2025.
Harvard physicist Cumrun Vafa echoed this sentiment: “The assumption that one can calculate dark energy independently of dark matter is misguided. This common presumption among cosmologists, including the DESI team, has led to physically unacceptable phantom behaviour.”
DESI’s revelations are not the sole impetus for reevaluating the relationship between dark energy and dark matter. Teixeira’s recent study illustrates how allowing for their interaction might alleviate one of cosmology’s most enduring challenges: the Hubble tension.
Unraveling the Dark Universe
The Hubble tension refers to the discrepancy in the current rate of the universe’s expansion, known as the Hubble constant. This value can be measured through the light of the primordial universe, allowing us to observe its growth during its infancy. Alternatively, it can also be gauged through contemporary phenomena, such as supernova explosions, which reveal the current expansion rate.
Under the standard cosmological model, the expansion rate should remain consistent regardless of the measurement method employed. However, recent discoveries indicate that the early universe’s expansion rate differs from that of the more recent universe by approximately 9%. This discrepancy has sparked debates within the cosmological community regarding whether it stems from systematic errors or signifies new physics.
Teixeira and her co-authors propose that in a scenario where dark energy and dark matter interact, what initially appeared to be a crisis due to disparate expansion rates transforms into a predictable outcome.
Khoury argues that if dark energy and dark matter indeed interact, it could imply a shared origin. Ongoing investigations into string theory—the concept that our universe, at its most fundamental level, comprises vibrating strings—hint at a potential link between these ideas. In 2019, building on the premise of naturally varying dark energy, Vafa and collaborators concluded that the mass of dark matter particles might also vary over time. This line of inquiry led them to propose in 2022 that dark matter and dark energy could be connected through what is referred to as a dark dimension.
The Implications of a Dark Dimension
String theory posits the existence of six or seven additional dimensions beyond the three spatial dimensions and the temporal dimension we are familiar with. It is theorised that these dimensions are minuscule, approaching the Planck scale (10⁻³⁵ metres), yet researchers propose that the dark dimension could be significantly larger, on the order of a micron (10⁻⁶ metres).
Gravitons, hypothetical particles responsible for transmitting the force of gravity, could seep into this enlarged dark dimension. If so, they would acquire mass and transform into what are known as dark gravitons. While these massive gravitons would reside in the dark dimension, their gravitational effects might be observable in other dimensions, fulfilling the role typically attributed to dark matter.
In this theoretical framework, “there exists a natural coupling between dark energy and dark matter,” noted Georges Obied, a physicist at the University of Chicago. Changes in the size of the dark dimension would impact both dark energy and dark matter.
In a July 2025 paper, Obied, Vafa, and collaborators discovered that the scenario proposed in 2019 was consistent with DESI’s data. Their model predicts that both the intensity of dark energy and the mass of dark matter will diminish over time, and that the rate of change in dark energy will be proportional to its energy density. Given that astrophysical measurements indicate an extraordinarily low energy density of dark energy, “it will not change rapidly,” Vafa stated.
“It is not surprising that we have not observed it until now, because the rate of change is minuscule. We had to wait for the entire age of the universe to detect such a small effect,” he added. If dark matter is indeed linked to dark energy, then dark matter particles might interact with one another through a new long-range force, independent of gravity. The silver lining, according to Obied, is that “there could be astrophysical methods to test this.”
Future Investigations and Their Significance
Notably, two physicists, Marc Kamionkowski, currently at Johns Hopkins University, and Michael Kesden from the University of Texas at Dallas, have previously explored such a test. In a 2006 paper, they envisioned a sequence of events where two galaxies approach one another, with the gravity of one galaxy attracting the other. If dark matter exerts a stronger gravitational pull on other dark matter than on ordinary matter, a unique “tidal tail” would form—a stream of stars, gas, and dust trailing behind one of the galaxies.
Kesden and Kamionkowski sought this effect and found no evidence of it, allowing them to set an upper limit on the potential strength of this additional attractive force. This upper limit was approximately 20 times greater than the value proposed by Vafa’s team, suggesting that the predicted value lies within the observable range. “It’s intriguing that we are now finding connections between that rather abstract work and observational and experimental efforts,” remarked Kamionkowski.
While the alignment of a prediction derived from string theory calculations with astrophysical evidence does not confirm the validity of these string models, any correlation between string theory and experimental findings is gratifying for Vafa, who has spent the past four decades striving to transition the theory from a purely conceptual realm to one capable of generating testable predictions.
Obied,