EVERY 250m years the sun, with its entourage of planets, completes a circuit of the Milky Way. Its journey around its home galaxy, though, is no stately peregrination. Rather, its orbit oscillates up and down through the galactic disc, the place where most of the galaxy’s matter is concentrated. It passes through that disc once every 30m years or so.

This fact has long interested Michael Rampino of New York University. He speculates that it could explain the mass extinctions, such as that of the dinosaurs and many other species 66m years ago, that life on Earth undergoes from time to time. Palaeontologists recognise five such humongous events, when up to 90% of species have disappeared. But the fossil record is also littered with smaller, though still significant, blips in the diary of life.

Many hypotheses have been put forward to explain these extinctions (they may, of course, not all have the same explanation). The two with most support are collisions between Earth and an asteroid or comet, and extended periods of massive volcanism. Dr Rampino observed some time ago that collisions might be triggered by gravitational disruptions of the Oort cloud, a repository of comets in the outermost part of the solar system. That would send a rain of them into the part of space occupied by Earth. This has come to be known as the Shiva hypothesis, after the Hindu god of destruction.

Oort-cloud disruption, Dr Rampino assumed in the past, would have been caused by passing stars, such as the encounter, 70,000 years ago, between the solar system and a star known as Scholz’s star which was reported in Astrophysical Journal Letters on February 12th. Now he is not so sure. In his latest paper, just published in the Monthly Notices of the Royal Astronomical Society, Dr Rampino speculates that the real culprit may be not stars, but dark matter—and that this might explain the volcanism as well.

Dark matter is stuff whose nature remains unknown. It is more than five times as abundant as the familiar matter that atoms are made of, but tends to interact with atomic matter only through gravity. The effect of this gravitational pull permits astronomers to measure where dark matter is, and they thus know that in the Milky Way it is concentrated, like atomic matter, in the disc. Add the effects of dark matter to those of atomic matter as the solar system passes through the galactic disc, Dr Rampino observes, and disruption of the Oort cloud is even more likely.

He is not the first person to have this idea. But he also argues that mutual gravitational attraction between dark matter and Earth will cause dark matter to accumulate at the planet’s core. And that is where things get interesting, for some theories about what dark matter is made of suggest its particles are their own antiparticles. This, if true, would mean that when they meet they annihilate each other in a burst of energy. Concentrating those particles inside a body like Earth would make them more likely to meet. And those meetings would create quite a lot of heat—sufficient, Dr Rampino suggests, to cause abnormal amounts of volcanism.

This is all guesswork, of course. Even the original Shiva hypothesis is widely doubted. Some researchers, Dr Rampino among them, claim that extinctions do seem to happen every 30m years or so. Others disagree, and say they are irregular.

How much irregularity Dr Rampino’s hypothesis can tolerate is unclear. There is bound to be some. Individual comet strikes are random events, even if the likelihood of one happening varies on a predictable cycle, and the detailed make-up of Earth’s interior at any given moment will affect exactly when eruptions happen. It may be too much to expect a clean pattern, with all that variation around.

But if Dr Rampino does turn out to be right, it will be an example of how important dark matter, ethereal though it be, really is. There is a tendency, even among physicists, to refer to atomic matter as normal and dark matter as strange. This would be a reminder that it is dark matter, not the “normal” sort, which is actually in charge.