National Geographic News
A diagram shows the evolution of the universe.
A time line shows the universe to be ever expanding and accelerating—traits often ascribed to dark energy.

Illustration courtesy WMAP Science Team, NASA

Ker Than

for National Geographic News

Published February 15, 2012

A powerful repulsion between normal matter and hidden pockets of antimatter could be an alternate explanation for the mysterious force known as dark energy, according to a controversial new theory.

In 1998 scientists discovered that the universe is not only expanding but that its expansion is accelerating.

This totally unexpected behavior has been called the "most profound problem" in physics, because our current understanding of gravity says that attractions between mass in the universe should be causing the expansion to slow down.

The leading theory to explain the accelerating expansion is the existence of a hypothetical repulsive force called dark energy. (Related: "New Galaxy Maps to Help Find Dark Energy Proof?")

But in the new study, Massimo Villata, an astrophysicist at the Observatory of Turin in Italy, suggests the effects attributed to dark energy are actually due to a kind of "antigravity" created when normal matter and antimatter repel one another.

"Usually this repulsion is ascribed to a mysterious dark energy that would uniformly permeate the cosmos, but nobody knows what it is nor why it behaves this way," Villata said in an email.

"We are replacing an unknown force caused by an unknown element with the repulsive gravity of the well-known antimatter."

(Related: "Dark Matter Is an Illusion, New Antigravity Theory Says.")

Antimatter Hiding in "Holes" in the Universe?

According to Villata, the keys to accelerated expansion lie in large-scale voids that are seen scattered throughout the cosmos.

These holes in our map of the universe—which can each be millions of light-years wide—are inexplicably empty of galaxies and galaxy clusters. The nearest hole to us is called the Local Void, bordering the Virgo supercluster of galaxies.

Villata thinks these voids harbor vast quantities of antimatter, which could even be organized into antimatter galaxies, complete with antimatter stars and planets.

(Related: "Antimatter Found Orbiting Earth—A First.")

All this antimatter doesn't emit radiation that can be detected by current sensors, making it effectively invisible, Villata said.

"There can be various reasons why antimatter in voids should be invisible, but we do not know which of them is the right one," Villata said. "Moreover, antimatter in laboratories could have different behavior, since it is 'immersed' in a world of matter."

(See "Antimatter Atoms Trapped for First Time—'A Big Deal.'")

While we can't see antimatter superstructures, we can observe their effects on our visible universe, Villata argues, because antimatter must repel the normal matter in galaxies, pushing them farther from one another.

Villata says his theory, which will appear in an upcoming issue of the journal Astrophysics and Space Science, has the potential to solve other cosmic mysteries, such as the universe's "missing antimatter" problem.

According to standard physics, matter and antimatter particles should have been created in equal amounts during the big bang. Yet the visible universe appears to be dominated by structures made up of normal matter.

To determine how much antimatter might be contained in the Local Void, Villata calculated how much would be needed to create a repulsive force strong enough to explain the so-called Local Sheet. This collection of normal matter, which includes our Milky Way and other nearby galaxies, is all moving at the same speed.

"If each void contains a mass of antimatter similar to that calculated for our Local Void ... then our universe would host an amount of antimatter equivalent to that of matter, and [there] would finally be a matter-antimatter symmetric universe," Villata said.

But Do Matter and Antimatter Repel?

While Villata's theory doesn't require mysterious forces created from nothing, it does rely on the untested assumption that matter and antimatter are mutually repulsive.

There is as yet "no [experimental] evidence that antimatter repels matter," said physicist Frank Close of the University of Oxford in the U.K., although, he added, plans are underway at the European Organization for Nuclear Research (CERN) in Switzerland to test the idea.

In fact, Dragan Hajdukovic, a physicist at CERN, recently proposed a separate antigravity theory that also relies on repulsion between matter and antimatter to explain dark energy and dark matter.

Hajdukovic called Villata's theory "an interesting idea," be he added that he disagrees with the hypothesis of a matter-antimatter symmetric universe.

"The major problem is why [such] big quantities of antimatter in the voids are not observed," Hajdukovic said.

In Hajdukovic's theory, antimatter particles spontaneously pop in and out of existence in the quantum vacuum, which is the name physicists give to seemingly empty space.

"I use the reality of the quantum vacuum. For a physicist, it is more natural and plausible," Hajdukovic said.

"In order to explain the invisibility of antimatter, proponents of a matter-antimatter symmetric universe would be forced to invoke an additional hypothesis"—such as the emission by antimatter of so-called advanced photons, which travel backward in time and so wouldn't be detectable to current instruments.

(Related: "Time Travel Impossible, Mini 'Big Bang' Hints.")

"It is not a good sign for a theory if one hypothesis immediately demands introduction of other hypotheses."

But study author Villata argues that the assumptions in his theory—including matter-antimatter repulsion and advanced photons—are predicted by well-established theories in physics.

In that sense, he said, there is "no introduction of other hypotheses."

Carl Prater
Carl Prater

Jef Franklin, you have hit the nail on the head. Villata has not gone far enough.  Just as matter's interaction with spacetime is attractive, antimatter's opposing interaction with spacetime must be repulsive.To all things, not just matter.This accounts for all observations and solves some of cosmology's biggest mysteries, not the least of which is baryogenesis.As the early universe cooled, matter and antimatter coalesced from the primordial energy.They would usually self annihilate, but antimatter with repulsive gravity would tend to diffuse while matter with attractive gravity would tend to concentrate.Eventually enough concentrated matter would gather to form galaxies, stars, and planets while antimatter would continue to diffuse and spread.Antimatter with repulsive gravity would be unable to form stars and therefore could never be fused into higher atomic number nuclei.So antimatter would exist primarily as antiprotons or anti-hydrogen if positrons were available.There could be versions of anti-helium too, but only as created during the earliest moments after the Big Bang.This is why no antimatter structures have been observed in the empty regions of intergalactic space.It also explains why gamma rays from annihilations are not observed since they could only occur at the particle or atomic level.Repulsive gravity of diffuse antimatter could also account for the accelerating expansion of the universe.As the space between galaxies grows, the repulsive gravity of diffuse antimatter would first neutralize, then begin to dominate over the attractive gravity of concentrated matter.

Jef Franklin
Jef Franklin

It's been said that the universe is a zero sum game. That at the moment of the big bang there was an equal amount of matter and antimatter. That is how Hawking explains how the universe may have came from nothing. But yet we have all this matter floating around around. If Hawking is right, then there must be large amounts of antimatter out there. How much can we really know about anti matter given that we cannot observe it in a natural state and only for very short periods of time when we do create it. So why can't we detect it. I think that it just may be because it is repulsive in nature. Just as matter is said to create a well in space that is gravity, antimatter creates a bulge in space that pushes everything away, possibly even other antimatter antiparticles. 

Rob Heusdens
Rob Heusdens

It's not anti matter but (gravitationally/inertially) NEGATIVE mass matter. It self repels (even though the force is attractive, but the acceleration is opposite to the force, since gravitational mass = inertial mass) and it would clutter around vast positive mass constellations.

Christopher Green
Christopher Green

 I like to see articles pointing out alternative theories to the expanding universe.  The whole 'dark energy' hypothesis isn't too much different in flavor from the the idea of 'aether' to explain how light propagated in empty space.  

It is just as likely that the laws of physics simply don't work exactly as we think of them now.  Perhaps the general theory of relativity is missing term that is near zero unless vast distances are involved.  That, at least would explain why the expansion of the universe cannot be derived from particle physics/quantum mechanics.  I'm not saying it is true, I'm just saying that we shouldn't automatically assume that our equations are correct and that the problem must be some undetectable form of matter or energy (in other words, a fudge factor:)  


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