Since the mid-90s, scientists have been trying to locate half of the universe's ordinary matter, which they believed was out there because of clues left over from the Big Bang, but it had never been seen.

"What we're talking about here is what scientists call baryonic matter, which is the normal stuff that you and I are made of," said Associate Professor Jean-Pierre Macquart, from the Curtin University node of the International Centre for Radio Astronomy Research.

Astronomy is full of missing stuff. Most of the universe is understood to be "dark matter" and "dark energy", which nobody has ever directly seen. But even more of a mystery for astronomers was that they couldn't find about half the ordinary matter in the universe.

"It's been a true embarrassment that we haven't been able to find it," said Professor Xavier Prochaska, an astronomer from the University of California, Santa Cruz.

'Putting to bed a cosmic mystery'

Astronomers had been looking at the universe using all sorts of different forms of light — from radio waves through to x-rays and visible light. None revealed the missing matter.

That was until they started to measure fast radio bursts — brief flashes of intense energy found racing across the universe — and discovered the missing matter hiding in the cold dispersed gas between galaxies.

"Intergalactic space is very sparse. The missing matter was equivalent to only one or two atoms in a room the size of an average office," Dr Macquart said.

Fast radio bursts (FRBs) were first detected in 2007, but are not fully understood.

As FRBs travel across the universe, they are dispersed and slowed by the matter they pass through. Each frequency of radio energy is slowed down differently by that matter.

Just like a prism spreads light into a rainbow of colours, each with a different frequency, the missing matter in the universe spreads the FRBs out into different frequencies.

By studying how far away the origin of the bursts were, and how much they had spread out by the time they reached Earth, the scientists were able to figure out how much matter was sitting between galaxies.

Repeating the process with six different FRBs coming from different parts of the universe, the team was able to figure out how — and where — the missing matter was. The results were published today in the top scientific journal Nature.

"It was a great feeling … to have that measurement, to know something that you think only a small number of people in the world know, to put to bed a cosmic mystery," Dr Macquart said.

WA's telescopes capture energy bursts

Seeing such tiny changes in FRBs required a mammoth effort, using a combination of massive radio and optical telescopes.

The team used the CSIRO's Australian Square Kilometre Array Pathfinder (ASKAP), — an array of 36 radio telescopes in the Mid West of Western Australia. Combined, the dishes equate to a single radio telescope with a 4,000 square metre dish.

They have the ability to look at a huge area of the sky in high resolution. That means without knowing where an FRB would come from, they were able to capture it, and analyse exactly how spread out each wavelength in it was.

Then the aptly named Very Large Telescope in Chile measured the distance between Earth and the galaxy that the FRB came from.

With those two pieces of information, the researchers were able to determine how much matter was in the otherwise empty space that each FRB passed through.

Many mysteries remain

The discovery still leaves much of the universe undetected. About 85 per cent of its matter is thought to be "dark matter" — that which is undetectable using ordinary methods.

"I like a mysterious universe. It means there is much more to be understood," Dr Macquart told the ABC.

"I think it's really exciting," said Professor Tara Murphy, an astronomer from the University of Sydney, who was not involved in the research.

"It's the first step to using fast radio bursts to probe the invisible universe," she said.

Dr Macquart said the next step was to get an intricate understanding of this newly found matter, using 100 or more FRBs.

"The thing that we've discovered is the atmosphere of the universe," he said. "It's the ecosystem in which galaxies live."

He said by understanding its precise structure, astronomers would be able to better understand how galaxies were born, and what happened to them when they died.

"It will be like a medical image of the universe," he said.

Below | Just like a prism spreads light into a rainbow of colours, each with a different frequency, the missing matter in the universe spreads the FRBs out into different frequencies.