Matter in the universe is distributed in a network of filaments known as the cosmic web. The biggest clusters of galaxies form nodes with dark matter and gas stretching in tendrils between them. Studying the gas is the difficult bit as it's not bright and shiny like galaxies, but astronomers have found a way to do it.

By using the light of rare quasar pairs – incredibly bright and distant objects – an international team of researchers have been able to detect the distribution of the original hydrogen atoms. As reported in Science, the variations were the size of galaxies (about 100,000 light-years) and will help scientists understand how the gas was distributed 2 billion years after the Big Bang.

“One reason why these small-scale fluctuations are so interesting is that they encode information about the temperature of gas in the cosmic web just a few billion years after the Big Bang,” co-author Dr Joseph Hennawi, from UC Santa Barbara, said in a statement.

There’s about one atom of hydrogen per cubic meter in intergalactic space, too tenuous for a direct observation. Astronomers looked at how the light of the quasars are absorbed by this gas, and by using quasars in pairs, the team was able to detect small differences.

“One of the biggest challenges was developing the mathematical and statistical tools to quantify the tiny differences we measured in this new kind of data,” explained lead author Dr Alberto Rorai of Cambridge University.

^{The light from quasar pairs reach Earth, although some were absorbed by the gas in the cosmic web, Springel et al. (2005) (cosmic web) / J. Neidel, MPIA}

The analyzed data was compared to the supercomputer models of the universe – one that requires the existence of dark matter and dark energy – and what the team observed matches the theoretical expectations of the standard cosmological model.

Advanced computing is at the very core of this research, and was not just used for analysis and comparison. Finding the targets was also incredibly difficult, with the the team using artificial intelligence algorithms known as machine learning to find the quasar pairs.

“Pairs of quasars are like needles in a haystack,” added Hennawi. “In order to find them, we combed through images of billions of celestial objects millions of times fainter than what the naked eye can see.”   

Quasars are galaxy phases that happen when a supermassive black hole is going through a feeding frenzy, becoming incredibly bright. These very luminous periods only last for a brief chunk of a galaxy’s life, so astronomers finding two in close proximity is really difficult.