Lewis Dartnell in The Telegraph:
Dark matter is one of the most intensely studied subjects in particle physics and cosmology today. It’s certainly curious stuff, behaving in ways unlike anything in our everyday experience. Indeed, dark matter has never actually been isolated or studied in the lab. Its very existence has only been inferred indirectly by its influence on things we can see. But what is dark matter? When astronomers look at a rotating galaxy, they can work out the mass of the stars and gas clouds within it, and thus the strength of that galaxy’s gravity, as well as how fast it is spinning and so how much gravitational force is required to hold it all together. The problem is that the two figures don’t match. Galaxies like our own apparently contain far more mass than the visible matter of stars and nebulae, or else they would have flung themselves apart in their rapid twirl. The implication is that there must also be something unseen, dark matter, the gravity of which holds it all together. We infer dark matter, as Lisa Randall puts it rather wonderfully in Dark Matter and the Dinosaurs, in the same way we would deduce the invisible presence of a celebrity in the midst of an excited, jostling crowd.
…The history of life on Earth is punctuated by mass extinctions, including that notorious asteroid or comet collision 65 million years ago. Various studies over recent decades have looked for a periodicity in the impact rate – the possibility that impacts on the Earth come in regular waves – and have hypothesised triggers, such as the elongated orbit of an unseen Planet X or perhaps a companion star to the Sun, dubbed Nemesis. A huge swarm of icy bodies encircle the Sun on the very outer edges of the solar system, known as the Oort cloud; if some of these were to have their orbit disturbed by an external gravitational influence, they could swoop down into the inner solar system as comets, and potentially slam into the Earth. But none of these proposed triggers have stood up to further study. Could there be a link, Randall posits, between the rate of impacts on the Earth and dark matter in the galaxy? Our galaxy, the Milky Way, is a great rotating spiral of stars, and as our sun orbits the centre, it also bobs up and down through the flat plane of the galaxy: our solar system’s overall motion around the galaxy is like that of a fairground carousel horse. It is this motion, claims Randall, that could cause regular waves of impacts on Earth.