Australian astronomers have long been contributing to our understanding of a strange cosmological phenomenon—the Universe’s missing matter.
In the early 1970s, Ken Freeman of the Australian National University (ANU) determined that spiral galaxies must contain more matter than we can see. He postulated that dark matter—an invisible material first proposed 40 years earlier—must make up at least half the mass of these galaxies. Now, patches of dark matter are thought to be scattered across the Universe, playing a major role in holding galaxies and groups of galaxies together.
Ken calculated the speed of rotation of disc-shaped galaxies, and found that they were spinning too fast for the amount of matter we could see in them. “They should have flung themselves apart; so something else must be there, something unseen but with lots of mass, holding them together,” he says.
Ideas on what the ‘something else’ could be have ranged from the very large to the very small: such as swarms of black holes, or burned-out stars or planets, to clouds of tiny neutrinos or other exotic particles.
The large candidates have been mostly eliminated, and work is now focused on the sub-atomic end of the scale. Some of this work is being done in particle physics laboratories, such as the Large Hadron Collider in Europe.
But astronomers are on the scent too. If dark matter is made of tiny particles, they might sometimes collide with other particles within galaxies and annihilate themselves, emitting radiation that can be detected by telescopes.
In addition, Australian observatories are scanning galaxies at radio wavelengths to get a better picture of the spread of normal matter. From that, astronomers can infer the extent of the still-unknown dark matter.