via New Scientist.
Dark matter, believed to be the second most prevalent source of mass in the Universe after Dark Energy has taken an extra turn for the complex, according to new research.
Dark Matter first came about when astronomers looked at the rotation of stars in galaxies. To a good approximation, they seemed to mostly be orbiting the galactic centre at the same angular velocity – ie as if they were all painted onto a solid disc, rather than being a swirl of gas that would rotate slower at the edges than in the centre due to the extra energy required to keep the edges in line. Further evidence that this was due to something unseen being present rather than a gravitational effect came in the form of clusters of stars in the galactic halo. These globular clusters are like islands of stars separated from the main disc, but they too orbited the galactic centre as if they were set into an invisible glass ball, whatever affected stars in the enormous flat disc also had an equal effect on isolated pockets of stars far from it.
Studies into how Dark matter must act to create the conditions seen suggest that it is cold (particles aren’t zipping around close to the speed of light) and composed of Weakly Interacting Massive Particles, that is particles that interact through gravity and no other means. However, the no other means bit has come under some scrutiny.
Research done by astrophysicists at the University of St Andrews in the UK has probed further into the effect of dark matter. It had been discovered that rather than clumping together into a ball that is denser at the centre than further out, dark matter attains a uniform density around the cores of galaxies. The new research looked into how dark matter acted outside of this core and found that when the density fell to one quarter of this core density, dark matter stuck to a five to one ratio to ordinary matter, that is it remained at five times whatever the density of stars and gas was.