Seeing dark matter in a new light

A small team of astronomers from Swinburne University of Technology has found a new way to ‘see’ the elusive dark matter halos that surround galaxies.

Our Universe is filled with many billions of galaxies. To our eyes – and in optical telescopes – these galaxies appear as collections of millions or even trillions of stars. However, this is just the tip of the iceberg.

One of the big puzzles of cosmology is that our telescopes only see a small fraction of the total mass that exists in the Universe. 

“Most – about 85 per cent – of the mass in the Universe is effectively invisible,” says Swinburne PhD candidate Pol Gurri, who led the new research. “Unlike ordinary matter, this dark matter does not produce, absorb, or reflect light: it only interacts with the rest of the Universe via gravity.”

So how can you measure what cannot be seen? The key is to measure the effect of gravity that the dark matter produces.

“It’s like looking at a flag to try to know how much wind there is. You cannot see the wind, but the flag’s motion tells you how hard the wind is blowing,” Mr Gurri explains.

The new research focuses on an effect called weak gravitational lensing, which is a feature of Einstein’s general theory of relativity.

“The dark matter will very slightly distort the image of anything behind it,” says Associate Professor Edward Taylor, who was also involved in the research. “It is a bit like looking at a table through the base of a wine glass.”

ANU 2.3 metre telescope. Credit: Australian National University