First Image Ever of the Dark Matter Web that Connects Galaxies –“A Web-Like Superstructure Predicted for Decades”




“For decades, researchers have been predicting the existence of dark-matter filaments between galaxies that act like a web-like superstructure connecting galaxies together,” said Mike Hudson, a professor of astronomy at the University of Waterloo. “This image moves us beyond predictions to something we can see and measure.”

The composite image shown below, which combines a number of individual images, confirms predictions that galaxies across the universe are tied together through a cosmic web connected by dark matter that has until now remained unobservable. In the image the locations of bright galaxies are shown by the white regions and the presence of a dark matter filament bridging the galaxies is shown in red.


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Dark matter, a mysterious substance that comprises around 25 per cent of the universe, doesn’t shine, absorb or reflect light, which has traditionally made it largely undetectable, except through gravity.

As part of their research, Hudson and co-author Seth Epps, a master’s student at the University of Waterloo at the time, used a technique called weak gravitational lensing, an effect that causes the images of distant galaxies to warp slightly under the influence of an unseen mass such as a planet, a black hole, or in this case, dark matter. The effect was measured in images from a multi-year sky survey at the Canada-France-Hawaii Telescope.

They combined lensing images from more than 23,000 galaxy pairs located 4.5 billion light-years away to create a composite image or map that shows the presence of dark matter between the two galaxies. Results show the dark matter filament bridge is strongest between systems less than 40 million light years apart.

“By using this technique, we’re not only able to see that these dark matter filaments in the universe exist, we’re able to see the extent to which these filaments connect galaxies together,” said Epps.

The image at the top of the page shows the supercomputer simulation project called EAGLE (Evolution and Assembly of GaLaxies and their Environments) confirms and shows in great detail what has long been suspected. The tiny fluctuations in density in the early universe were magnified by gravity, with dark matter slowly congealing into denser clumps and filaments which then drew in hydrogen and helium gas where it formed into the first stars and galaxies.

The EAGLE simulation starts before the first stars and galaxies formed. Measurements from the COBE, WMAP, and most recently, the Planck space telescopes all show the universe was uniform to one part of 100,000. Other key input parameters to EAGLE include the density of dark matter, the density of “normal” matter made of hydrogen and helium, and the mysterious cosmological constant that causes the expansion of the universe to accelerate.

Using known physics and the composition of the early universe, EAGLE models the formation of galaxy clusters in a volume of of 300 million light-years on a side. That’s large enough to contain 10,000 galaxies of the size of the Milky Way or bigger. It also keeps track of more than 7 billion particles over hundreds of millions of years.

The Daily Galaxy via Royal Astronomical Society and University of Waterloo

Image credit: S. Epps & M. Hudson / University of Waterloo


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