Physicists have calculated how much matter exists in the universe, but there's a big chunk they haven't been able to find. Recently, two separate groups of researchers say they've managed to spot some of the "missing" matter.

Astrophysicists from the University of British Columbia and the University of Edinburgh independently reported that they found the missing baryonic matter in filaments of gas stretching between galaxies using data from the European Space Agency's Planck telescope.

Both groups released their findings on the physics pre-print server arxiv.org late last month. The papers have not yet been published in a scientific journal and are still undergoing peer review. But that didn't stop them from generating splashy headlines, such as "Half the universe's missing matter has just been finally found" and "Mystery of the universe's missing matter finally solved by scientists."

ESA’s Planck space telescope, seen in an artist's impression, was launched in 2009 and operated until 2012. It provided much more detailed information about the cosmic microwave background than was ever available before. (C. Carreau/ESA)

According to physics theory, the universe is made of dark energy, dark matter and "normal" matter (the kind that humans, the Earth and the stars are made of). Normal matter is made of baryon particles, which include the protons and neutrons that form atoms.

You may have heard that scientists have a hard time finding dark matter because it's invisible. What you may not know is that most of the normal matter in the universe also hasn't been detected.

"Most of it — basically 90 per cent of it — we don't see because they're not in the form of a star," says Niayesh Afshordi, an astrophysicist and professor at the Perimeter Institute for Theoretical Physics and the University of Waterloo. "They are in some tenuous gas or plasma in between the stars."

That material is thought to form filaments that join together into a "cosmic web" and is sometimes known as warm-hot intergalactic medium or "WHIM." It's hard to detect because it's too diffuse and not hot enough to glow brightly enough for us to see with a telescope.

So Hideki Tanimura, then a PhD physics student at UBC, and Anna de Graaff, a master's student in astrophysics at the University of Edinburgh, and their collaborators tried to spot the filaments indirectly.

Subtle colour change

They used recent data from the European Space Agency's Planck telescope, which mapped the cosmic microwave background, the afterglow left by the Big Bang that's thought to have given birth to the universe about 14 billion years ago. The particles that make up that afterglow are very, very cold. But if they happen to hit something comparatively hot, such as electrons inside the filaments between galaxies, they change colour very slightly.

"The signal is very subtle," said Afshordi, adding that it was almost undetectable until very recently, when data from the very sensitive Planck telescope became available.

The gravity of a luminous red galaxy has gravitationally distorted the light from a much more distant blue galaxy, an effect called gravitational lensing. Hideki Tanimura looked for filaments of gas between pairs of luminous red galaxies. (ESA/Hubble/NASA)

Tanimura and de Graaff both looked for that colour change between pairs of galaxies, where one might expect to find filaments. The difference was that they chose different types of galaxies at different distances from Earth.

Both reported finding the filaments.

"I was very surprised," said Tanimura, who graduated in September and is now at the Institute of Space Astrophysics in Orsay, France.

That's because when he calculated the density of the gas in the filaments, it was lower than he expected — lower than he thought would be detectable.

'It confirms my result'

He hadn't been aware that de Graaff was working on a very similar study until he saw her paper.

"I think it's great," he added. "We have consistent results … It confirms my result."

De Graaff declined to be interviewed by CBC News about her study. She said it was under embargo now that it has been submitted to a journal, even though it's still available online.

For me, they've never been missing. - Dick Bond, University of Toronto

De Graaff's paper estimates that the material her team found could account for 30 per cent of the baryons in the universe.

Tanimura's paper doesn't make an estimate for the amount of baryons his team may have found. He said that's because you need to make an assumption about the temperature of the gas in order to get such an estimate.

Afshordi thinks that's prudent — all the researchers could see, he says, was the heat from the gas. The same amount of heat could come from a very small amount of mass that's very hot or a much larger amount of mass that's cooler: "If it's a little mass, you still have missing baryons."

He also thinks that the two studies don't do enough to show that the heat comes from filaments of gas between galaxies — which has never been seen — rather than matter from the edges of galaxies, which astronomers have already accounted for.

J. Richard Bond, an astrophysicist and cosmologist at the University of Toronto and the Canadian Institute for Advanced Research is working on techniques similar to those used by Tanimura and de Graaff. He says there's "no question" their teams detected something.

He expects at least some of it is indeed from outside galaxies. That's because astrophysicists have calculations that show it should be there — it was just a matter of developing techniques sensitive enough to detect it.

So even though the matter is referred to as "missing baryons" — "for me, they've never been missing," Bond said.

Afshordi agrees that confirming where in the universe the missing baryons are doesn't solve the biggest mystery.

"The mystery is in understanding what's happening to most of the matter."