Researchers come up with new method to ‘see’ dark matter

Scientists know that just over a quarter of all that exists in the universe is in the form of dark matter, something we can’t see directly. We know that dark matter must be there because of the ways that galaxies move, which shows they have a lot more mass than we can observe. So we call the remaining unknown mass dark matter.

But how do you study something you can’t see? The next generation of dark matter instruments will use new techniques and extremely accurate hardware to measure the movements of distant galaxies. But for now, a small group of astronomers from the Swinburne University of Technology have come up with a way to “see” dark matter using current telescopes.

The method works by looking for the gravitational effects of dark matter, rather than the presumed particles themselves. “It’s like looking at a flag to try to know how much wind there is,” lead author Pol Gurri explained in a statement. “You cannot see the wind, but the flag’s motion tells you how strongly the wind is blowing.”

Artist's impression of a galaxy surrounded by gravitational distortions due to dark matter. Galaxies live inside larger concentrations of invisible dark matter (coloured purple in this image), however the dark matter's effects can be seen by looking at the deformations of background galaxies.
Artist’s impression of a galaxy surrounded by gravitational distortions due to dark matter. Galaxies live inside larger concentrations of invisible dark matter (colored purple in this image), however the dark matter’s effects can be seen by looking at the deformations of background galaxies. Swinburne Astronomy Productions - James Josephides

The research uses a technique called weak gravitational lensing, in which they observe distant galaxies and wait for another galaxy to pass between it and us. When this happens, the in-between galaxy bends the light waves from the distant galaxy due to its gravity. “The dark matter will very slightly distort the image of anything behind it,” explained Associate Professor Edward Taylor, who was also involved in the research. “The effect is a bit like reading a newspaper through the base of a wine glass.”

This technique has been used before to investigate dark matter. But it usually requires highly accurate telescopes, which measure the shape of the distant galaxies. The team’s innovation was to look at how galaxies rotate instead.

“Because we know how stars and gas are supposed to move inside galaxies, we know roughly what that galaxy should look like,” Gurri said. “By measuring how distorted the real galaxy images are, then we can figure out how much dark matter it would take to explain what we see.”

This means that even older telescopes, like the ANU 2.3m Telescope in Australia, can be used to “see” dark matter, in a more accurate way than if they were not looking at rotation.

“With our new way of seeing the dark matter,” Gurri said, “we hope to get a clearer picture of where the dark matter is, and what role it plays in how galaxies form.”

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