WATERLOO — University of Waterloo researchers have released the first composite picture of the filaments that connect the cosmic web.

The image of a dark matter bridge that connects two galaxies was captured by University of Waterloo astronomy professor Mike Hudson and Seth Epps, a former master's student at the university. Their research was published Wednesday in the Monthly Notices of the Royal Astronomical Society.

"It's very cool," Hudson said in an interview. "I just find it amazing that I'm looking at a picture of dark matter. It's just incredible that we can actually produce these maps."

Dark matter is a mysterious substance that makes up slightly more than a quarter of all matter in the universe. It doesn't absorb, emit or reflect light, making it extremely difficult to spot, and has so far only been detected through gravity.

The composite image, made up of several individual images, confirms predictions that galaxies are tied together through a cosmic web connected by dark matter that has until now remained unobservable.

Other researchers had predicted the existence of the filaments of dark matter that connect galaxies, "but the evidence for them was very indirect," Hudson said.

"What is new from our work is that in addition to detecting them, we've been able to make this picture and measure them. We know how wide the filament is on average, and how much mass it has."

Hudson and Epps used a technique called weak gravitational lensing. It's an effect that causes the images of distant galaxies to distort slightly from an unseen mass such as a planet, a black hole or, in this case, dark matter. The effect was measured in sky survey images from March 2003 to November 2008 at the Canada-France-Hawaii Telescope.

The researchers combined lensing images from more than 23,000 pairs of galaxies 4.5 billion light-years away to create a composite image that shows the presence of dark matter connecting two galaxies.

"Galaxies and clumps of galaxies don't sit in empty space like little balls," Hudson said. "They're joined together with a three-dimensional spider web" of dark matter.

Despite scientists' use of terms like filaments, the scale is enormous. The filaments of dark matter strung between the pairs of galaxies were 40 million light-years long, far larger than the size of our own galaxy.

The ability to measure the filaments opens up a whole new area of research, Hudson said, and scientists are eagerly awaiting much more precise data sets expected within the next few years.

"The existence of dark matter is quite well established, but knowing it's there is quite different than being able to measure it," Hudson said.

"What we're doing now takes this to a whole new level of understanding of its properties. We'll be able to measure the mass more accurately, measure different lengths of filaments, and measure the change in the filaments over cosmic time," he said.

The image was produced through the efforts of a large team working with the data gathered from the Canada-France-Hawaii Telescope, he said. It took more than 10 years to catalogue and analyze the distortions in the lensing images and a couple more to extract the image of the filaments.

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That first sight of the image of the filament was a thrill, Hudson said. "I was ecstatic when that picture sort of popped out. It was quite fantastic.

"As scientists, this is what we live for."