Breakthrough gravity discovery challenges the existence of dark matter

In the vast universe, galaxies rotate in ways that don’t make sense if only visible matter is considered. For almost a century, scientists have tried to explain this by searching for a hidden force called dark matter. It’s been an idea so accepted that entire models of the universe rely on it. But what if something else entirely was responsible for the mysterious pull that holds galaxies together?

That’s the question raised by physicist Dr. Richard Lieu at The University of Alabama in Huntsville. In a paper published in the Monthly Notices of the Royal Astronomical Society, Lieu offers a theory that could challenge one of the biggest assumptions in astrophysics. His idea: gravity can exist without any mass at all.

The study explores a different solution to the same equations that normally describe gravity—both in Newtonian theory and in general relativity. These equations link mass with the gravitational force it creates. Lieu focused on what’s known as the Poisson equation, a simplified form of Einstein’s field equations used for describing gravity in weaker fields, like those around galaxies.

This equation typically has one well-known solution: gravity that weakens with distance, created by mass. But there’s another, lesser-known solution that’s often ignored. It can also create an attractive force but doesn't come from any actual matter.

Lieu took this second solution seriously. “My own inspiration came from my pursuit for another solution to the gravitational field equations of general relativity,” he explains. He was motivated by frustration that the idea of dark matter has lasted so long without direct evidence.

Instead of invisible matter, Lieu believes the extra gravity may be coming from structures in space known as topological defects.

Topological defects are regions in the universe where the structure of space changes suddenly. These can form during a phase transition—when the state of matter changes across the entire universe, like during its earliest moments.

Lieu’s model focuses on shell-shaped topological defects that may have formed during such cosmic events. These shells would have two extremely thin layers: an inner layer of positive mass and an outer layer of negative mass. The sum of the two layers is zero. So even though they create a strong gravitational pull, they technically don’t add any measurable mass.

“The shells in my paper consist of a thin inner layer of positive mass and a thin outer layer of negative mass,” Lieu says. “The total mass of both layers — which is all one could measure, mass-wise — is exactly zero, but when a star lies on this shell it experiences a large gravitational force pulling it towards the center of the shell.”

This setup means you can have gravity without mass—something long believed to be impossible. These shells wouldn’t just hold galaxies together. As Lieu shows, they could even bend light.

Gravitational lensing is one of the strongest pieces of evidence for dark matter. When light from a distant object, like a quasar or galaxy, passes by a massive object, it bends. The amount of bending matches how strong the gravitational field is, not just how much visible matter is present.

Lieu’s model shows that a series of massless shells, arranged concentrically like layers of an onion, could deflect light in the same way as an isothermal dark matter sphere. Each shell slightly bends a light ray passing through it. The total effect across many shells produces the same deflection as if there were a large amount of invisible mass in the center.

“Gravitational bending of light by a set of concentric singular shells comprising a galaxy or cluster is due to a ray of light being deflected slightly inwards — that is, towards the center of the large-scale structure,” says Lieu. “The sum total effect of passage through many shells is a finite and measurable total deflection which mimics the presence of a large amount of dark matter.”

Lieu points out that both the way stars orbit within galaxies and the bending of light are used to measure gravity on cosmic scales. His argument is simple: if these measurements can be explained without adding any dark matter, why keep assuming it’s there?

One of the central puzzles in astrophysics is why stars in galaxies don’t slow down as they move farther from the center. Based on Newton’s laws, stars farther out should move more slowly, just as planets do in the solar system. Instead, they move at a nearly constant speed, creating a “flat” rotation curve.

Lieu’s solution can create that kind of flat rotation without any mass. When solving the Poisson equation for a special kind of source—a topological defect shaped like a thin shell—you get a gravitational field that causes objects to orbit at a constant speed, no matter how far they are from the center.

That result challenges the very link between gravity and mass. It shows how massless sources—like Lieu’s shell structures—can generate a force strong enough to keep galaxies spinning in a stable, flat pattern.

Lieu’s work doesn’t prove that dark matter doesn’t exist. He admits that further steps are needed to understand how these shell structures form and evolve over time. “It is unclear presently what precise form of phase transition in the universe could give rise to topological defects of this sort,” he says.

There are still open questions about stability and formation. Could these shells have started as flat planes or long cosmic strings, later twisted into spherical forms by angular momentum? How can we detect them directly? And do they exist at the scale needed to match current observations of galaxy dynamics?

Yet the strength of the theory lies in its ability to explain what we see—without asking for more than what we can detect. It also echoes historical challenges in science where long-held beliefs had to be revised or abandoned in the face of better models.

Lieu says, “Of course, the availability of a second solution, even if it is highly suggestive, is not by itself sufficient to discredit the dark matter hypothesis — it could be an interesting mathematical exercise at best. But it is the first proof that gravity can exist without mass.”

If his ideas are confirmed through observation and further theoretical work, they could reshape modern cosmology. For now, the theory offers a bold step forward in a debate that has spanned generations.

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