Scientific American has an interesting story that discusses mass extinctions and their causes.  Back in the 1980s, University of Chicago paleontologists David Raup and Jack Sepkoski found evidence for a 26 million year periodicity in the largest mass extinctions in earth’s history.  Since then, other studies have found a similar period of around 30 million years for extinction events.  There might also be an alignment of impact and volcanic activity to coincide with the extinctions.  For lack of a plausible mechanism, these theories have languished on the scientific fringe.  Now, Michael Rampino, a geoscientist at New York University, thinks that dark matter might be that mechanism.

Dark matter is an invisible substance that scarcely interacts with the rest of the universe through any force other than gravity. Whatever dark matter is, astronomers have inferred there is quite a lot of it by watching how large-scale structures respond to its gravitational pull. Dark matter seems to constitute almost 85 percent of all the mass in the universe, and it is thought to be the cosmic scaffolding upon which galaxies coalesce. Many theories, in fact, call for dark matter concentrating in the central planes of spiral galaxies such as the Milky Way. Our solar system, slowly orbiting the galactic core, periodically moves up and down through this plane like a cork bobbing in water. The period of our bobbing solar system is thought to be roughly 30 million years. Sound familiar?

In 2014, the Harvard University physicists Lisa Randall and Matthew Reece published a study showing how the gravitational pull from a thin disk of dark matter in the galactic plane could perturb the orbits of comets as our solar system passed through, periodically peppering Earth with giant impacts. To reliably knock the far-out comets down into Earth-crossing orbits, the dark-matter disk would need to be thin, about one-tenth the thickness of the Milky Way’s visible disk of stars, and with a density of at least one solar mass per square light-year.