There isn't enough visible matter in the universe to account for the gravitational force that holds galaxies together. That's why scientists are searching for evidence of the missing "dark matter" that makes it possible. Now, one researcher has advanced an odd theory: dark matter is actually a galactic-scale electric field.

Over the decades, astrophysicists have proposed several candidates for dark matter, which they say accounts for nearly 85% of the mass of the universe, including: sterile neutrinos, WIMPs (weakly interacting massive particles), WIMPZILLAs, brown dwarfs and gravitinos. Another hypothetical dark matter particle, axions, might have recently been detected, though it could take years to confirm the discovery.

And now, we have another speculative idea to add to the dark matter menagerie. Physicist Stephen Reucroft has self-published a paper arguing that electrostatic forces could be preventing galaxies from flying apart. Flying in the face of current thought, he suggests that the extreme activity at the center of a galaxy is bound to propel electrons, and other negatively charged particles, towards the outer regions. This would give the galactic core a net positive electric charge, while the outskirts would become negatively charged. "In fact it is quite implausible that the core should remain electrically neutral," he writes.

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Reucroft addresses the question of how much of a charge would be required to generate the force capable of holding a galaxy together as it rotates. As the Physics arXiv Blog explains:

The answer is approximately 10^31 Coulombs at the core and an equal and opposite charge distributed throughout the galactic periphery. That is a significant amount of charge given that the definition of a Coulomb is the charge transported by a constant current of one ampere per second.

However, Reucroft says that on a galactic scale, this amount of charge is relatively little. He calculates that it would be equal to less than one part in 10^17 of the available charge in the galactic core. "It corresponds to one free proton for every ~10^11 cubic meters of galactic core volume," he says.

At the Solar System's location, that would produce a galactic electric field of approximately 1 volt per meter — an effect that would presumably be observable in ways other than on the orbits of stars, although Reucroft does not suggest how.

That's a provocative idea that offers an interesting alternative to dark matter. But it is also one that needs more work to produce some testable hypotheses.

Astronomers might also want to know how this electric field could influence the orbit of stars unless the stars themselves are negatively charged. For that, theorists will need to come up with a convincing charging mechanism.

EDIT: Steve Reucroft's paper was self-published on arXiv, and has not been peer-reviewed. In an earlier version of this story, we did not mention these salient facts.