NEAR the centre of the Milky Way lurks something at least as weird as dark matter, but definitely visible. Through their radio telescopes, astronomers can see a bright strand, 150 light years long and a couple wide, and kinked in two places. They call it the Snake.

Since its discovery in 1991 by Andrew Gray at the University of Sydney, the Snake has been a puzzle. Other “threads”, as they are known, have been found inside the Galactic Centre Lobe, a barrel-shaped re gion of space where the interstellar gas is slightly more rarefied than outside it. Among the many exotic theories of their genesis is that they are loops of magnetism--perhaps blown like smoke rings off newly-forming black holes--which bounced off the inside wall of the Lobe. But the Snake, being outside the Lobe, defies this explanation. Other theories, which did not depend on the Lobe, seemed capable of explaining smooth filaments, but not kinked ones.

Gregory Benford, of the University of California, Irvine, who proposed a competing theory before the Snake was found, has just updated it, and thinks it can account for these anomalies. His idea is that the Snake is an electrical discharge, like a vast lightning bolt--so vast that it just hangs in space, instead of disappearing as earthly lightning does.

Its source is a giant electrical dynamo. When something that conducts electricity sweeps through a magnetic field, a current starts to flow through it (this is the basis of a dynamo). Gas clouds in the middle of the galaxy are, everyone agrees, partly ionised: radiation flying through has battered electrons off some of the atoms in the gas. Because electrons are what carry electricity, the clouds should therefore be electrical conductors. Dr Benford reckons some of them could have built up an electric current as they moved through the strong magnetic field that pervades the centre of the galaxy. Once this had happened, the current could gradually have extruded itself from one of the denser areas of cloud, and followed it as it drifted through space--hanging on, in Dr Benford's words, like a lamprey.

However, this current itself twists around and around as it flows through the magnetic field, the way a torrent of water twists up a fire-fighter's hosepipe. That explains the kinks. Dr Benford reckons the Snake is actually something like a corkscrew shape seen side-on.

Such knowledge may not be entirely esoteric. Dr Benford thinks a better understanding of the Snake could give clues about how to handle similar (if smaller) structures on earth. That is something nuclear fusion researchers would love to do. To achieve their goal, they need to learn how to control ionised gases similar to those thought to compose the Snake.

But the Benford theory has not kicked its rivals down yet. Don Melrose, also at Sydney, points out that the gas in the middle of the galaxy might already be so strongly ionised, and thus such a good electrical conductor, that current would leak away rather than build up into a concentration like the Snake. Dr Melrose also thinks that some of the other theories, though they have shortcomings, do not rule out filaments with kinks.

If Dr Benford's theory is right, though, it also predicts that the Snake is slowly expiring. The radio signals that make it show up come from electrons flowing in it as they spiral through the galactic magnetic field. But this uses up energy, so the Snake is slowly glowing itself to death. Perhaps only its eventual disappearance will settle the debate.