“The quasars are back-lights,” Pieri told Reuters, and the way the gas in front of them absorbs some of the light allows astronomers to get a detailed picture of these distant clouds of gas known as the intergalactic medium.

The study is the first fruit from a five-year project started in 2009. The team, from the third Sloan Digital Sky Survey, expect to expand the survey with light from about 160,000 quasars by the end of the project.

“We’re essentially measuring the shadows cast by gas along a series of lines, each billions of light-years long,” said Will Percival, a cosmology professor the University of Portsmouth.

“The tricky part is combining all those one-dimensional maps. The problem is like trying to recognize an object from a picture that’s been painted on the quills of a porcupine,” he said.