A preprint study proposes a fifth dark dimension that controls both dark energy and dark matter.
This hypothesis offers a potential explanation for the universe’s most persistent mysteries. It suggests a single, evolving dimension governs the unidentified substance comprising most cosmic mass and the force driving accelerated expansion.
The theoretical origin
The concept stems from string theory, which posits that the universe contains dimensions beyond the four we perceive: time and three-dimensional space. Experts in the field have argued over recent years that one of these hidden dimensions could regulate dark matter and dark energy simultaneously.
Cumrun Vafa, a professor at Harvard University and co-author of the work, explained the mechanism during a call with 404 Media. He noted that the extra dimension is linked to the value of dark energy. Consequently, any change in the dimension’s state would alter the value of dark energy.
Challenging the standard model
When Vafa and his team developed this idea, the scientific consensus held that dark energy possessed a fixed value. This assumption is a central tenet of the standard model of cosmology, a well-corroborated framework explaining most universal phenomena.
Vafa recalled the resistance they faced. “We were scrutinized by our colleagues for predicting that string theory requires dark energy to change, whereas dark energy looks constant,” he said. “This is not good for our field because you are predicting string theory is inconsistent with observation.”
However, the assumption of a fixed value has been challenged by the Dark Energy Spectroscopic Instrument (DESI). Based at the Kitt Peak National Observatory in Arizona, the project released its first results in 2025.
To the surprise of many, the data hinted that the value of dark energy shifts over time. This finding aligns with the predictions made by Vafa’s team regarding a dynamic dark dimension.
“We didn’t write a model to fit the data, but rather we had a model based on theory,” Vafa stated. “Now that fits the data automatically.”
The team believes their approach is distinct from models constructed solely to match existing observations. “Our model is still the best model, as far as I know, that fits the DESI results,” he added.
How the dimension works
In their latest preprint, which is currently undergoing peer review, the authors expand on the mechanics of this possible dimension in light of the new data. String theory suggests that extra dimensions are “compactified” into subatomic scales, rendering them undetectable by current instruments.
The hypothetical dark dimension, however, may expand to a micron scale range. This size would allow gravitons—the hypothetical particles mediating gravity—to spill over into the dark sector. There, they would transform into dark matter particles.
Under this view, dark matter is not a new exotic particle. It consists of gravitons that exist in our familiar universe but have traversed into the dark dimension.
“We are using the extra dimension to our benefit to explain what dark matter is,” Vafa said. “We don’t need to introduce a new particle or particles. We are just saying it’s the usual gravity.”
As the dark dimension evolves, the mass of these particles changes. Vafa’s team predicts they are becoming lighter. Simultaneously, the value of dark energy weakens. In this way, two entities long thought unrelated arise from the properties of the same extra dimension.
Testing the hypothesis
While the concept is abstract, it can be tested. If a dark dimension exists, it should manifest as tiny deviations in Newton’s gravitational force at micrometer scales.
A team at the Institute for Quantum Optics and Quantum Information in Vienna is designing an experiment to check this, with results expected in the coming years.
Furthermore, if dark matter particles are merely gravitons in a different dimension, scientists should observe subtle hints of a “fifth force” in the dark sector. Researchers plan to test this by monitoring the movements of stars in interacting galaxies held together by dark matter halos.
When galaxies approach one another, the visible matter of each attracts the other. The dark matter of one galaxy also attracts the dark matter of the other. However, the presence of this fifth force creates an extra attraction between the dark matter components.
“There’s extra force between the dark matter compared to the visible matter because of this fifth force, so you get a more attractive force pulling the dark matter towards each other more than the visible part,” Vafa explained.
Vafa concluded that multiple lines of evidence could converge to confirm the theory. “Of course, the most exciting would be the confluence of all of these together, that somehow they agree. That would be really remarkable.”
What it means
For physicists, this study shifts the goalposts. Instead of searching for a new particle to explain dark matter, researchers can look for gravitational anomalies at microscopic scales or deviations in how dark matter interacts between galaxies. The next few years will determine if these theoretical predictions hold up against experimental data.
