Dark matter strands seen for the first time?

For the first time, astronomers may have seen the mysterious strands of dark matter that hide beneath the visible universe.

Scientists found a distant quasar illuminating a vast nebula of diffuse gas, revealing for the first time part of the network of filaments thought to connect galaxies in a cosmic web.

Using the 10-meter Keck I telescope in Hawaii, researchers at the University of California, Santa Cruz detected a very large, luminous nebula of gas extending about 2 million light-years across intergalactic space.

"This is a very exceptional object: it's huge, at least twice as large as any nebula detected before, and it extends well beyond the galactic environment of the quasar," said Sebastiano Cantalupo, first author of the paper.

The standard cosmological model of structure formation in the universe predicts that galaxies are embedded in a cosmic web of matter, most of which (about 84 per cent) is invisible dark matter, researchers said.

This web is seen in the results from computer simulations of the evolution of structure in the universe, which show the distribution of dark matter on large scales, including the dark matter halos in which galaxies form and the cosmic web of filaments that connect them.

Gravity causes ordinary matter to follow the distribution of dark matter, so filaments of diffuse, ionised gas are expected to trace a pattern similar to that seen in dark matter simulations.

Until now, these filaments have never been seen, researchers said.

In this study, the researchers detected the fluorescent glow of hydrogen gas resulting from its illumination by intense radiation from the quasar.

"This quasar is illuminating diffuse gas on scales well beyond any we've seen before, giving us the first picture of extended gas between galaxies," said J Xavier Prochaska, coauthor of the research.

"It provides a terrific insight into the overall structure of our universe," said Prochaska.The hydrogen gas illuminated by the quasar emits ultraviolet light known as Lyman alpha radiation.

The distance to the quasar is so great (about 10 billion light-years) that the emitted light is "stretched" by the expansion of the universe from an invisible ultraviolet wavelength to a visible shade of violet by the time it reaches the Keck telescope and the Low Resolution Imaging Spectrometer (LRIS) used for this discovery.

The study was published in the journal Nature.