Dark matter hints in widest-yet view of dark mystery

The survey dwarfs the previous largest map, shown at centre alongside the moon for comparison of size in the sky

Researchers have released the biggest images yet detailing dark matter, the mysterious substance that makes up 85% of the Universe's mass.

Each image, a billion light-years across, shows evidence of dark matter clumps scattered through the cosmos.

The team from the Canada-France Hawaii Telescope inferred the dark matter's existence by the way it bends light.

The images were presented at the 219th meeting of the American Astronomical Society in Austin, US.

The four images were taken at four different seasons of the year, each capturing a swath of the sky about as large as a palm held at arm's length.

They are a big step forward in understanding both dark matter itself, and the means by which dark matter influences the way normal matter clumps into the galaxies we see in the night skies.

Together, they represent the images of more than 10 million galaxies, whose light gives the only hints of the large-scale structure of dark matter.

"Light coming toward us from a distant galaxy is bent by the gravity of a lump of matter in the middle," explained Catherine Heymans of the University of Edinburgh.

"Einstein's theory of general relativity tells us that mass bends space and time, so when light comes toward us through the Universe, if it passes some dark matter, its light gets bent and the image we see gets bent and distorted," Dr Heymans told the meeting.

"Dark matter is leaving its signature on the images of very distant galaxies."

The survey is some 100 times larger than the previous largest map of the web of dark matter, gathered by the Hubble telescope's Cosmic Evolution Survey, or Cosmos.

In the new image, the full-scale distribution of vast clumps of dark matter can be seen around galaxy clusters, connected by wispy filaments of dark matter and trailing off to enormous voids where no matter exists.

Thankfully for theoretical astrophysicists, these images line up neatly with theory.

"Our theories of dark matter say that it should form a giant intricate cosmic web and that's exactly what we see in this data, a cosmic web that's housing the galaxies that we can see," Dr Heymans told BBC News.

Astronomy of scale

Dark matter at these huge, cosmological scales is only one part of the story, however, and Sukanya Chakrabarti of Florida Atlantic University presented work showing how the "dark matter halo" that surrounds individual galaxies can be characterised.

Building on work first presented at last year's meeting, Dr Chakrabarti showed how the ripples in the gas trailing behind spiral galaxies are giving an ever-better view of how dark matter is distributed around galaxies, and how it may influence how they form.

Rachel Mandelbaum of Princeton University said that the findings were significant, tackling the mystery on two fronts.

"Both of these results represent two important ways of studying dark matter but in very different regimes," she told the meeting.

"They're important in themselves, but they're also important as a proof-of-concept for the future, allowing us to see how powerful these methods will be with other data sets to do additional work."

And a great deal of work is still needed. Dr Heymans conceded that we still need to find out the nature of the dark matter particle, as well as discover more about the even more mysterious dark energy, which serves to drive the Universe's expansion even as dark matter works to draw things together.

"I'll be very honest with you, we don't know what the dark matter particle is, we don't know what this dark energy is coming from," she said.

"It's very widely believed that our final understanding of the 'dark universe' is going to have to invoke some new physics, something that will forever change our view of the Universe.

"What the team is presenting today are the first steps to reaching this goal."