Dark matter 'hairs' around Earth, illustration. The filaments of dark matter, called 'hairs', were proposed in a study in the Astrophysical Journal by Gary Prezeau of NASA's Jet Propulsion Laboratory. A 'hair' is created when a stream of dark matter particles goes through the planet. According to simulations, the hair is densest at a point called the 'root'. When particles of a dark matter stream pass through the core of Earth, they form a hair whose root has a particle density about a billion times greater than average. The hairs in this illustration are not to scale. Simulations show that the roots of such hairs can be 1 million kilometres from Earth, while Earth's radius is only about 6400 kilometres. Image published in 2015.