Half of ordinary matter in universe has long been 'missing.' Astronomers just found it.

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17-06-2025

Astronomers have long estimated that ordinary matter – basically, anything other than dark matter – makes up only a fraction of the known universe.
The conclusion stemmed from a complex calculation involving observed light left over from the Big Bang roughly 13.8 billion years ago. But there was one major problem: they had no clue where about half of it was.
Now, it seems as if a team of astronomers has finally tracked down that missing "ordinary" matter, which they discovered hiding as gas spread out in the vast expanses between galaxies. Revelations made possible by studying radio waves hurtling through space suggest that violent cosmic forces have played a role in the remote locations of almost all of the "missing" matter.
"The question we've been grappling with was: Where is it hiding? The answer appears to be: in a diffuse wispy cosmic web, well away from galaxies," Harvard University astronomy Liam Connor, lead author of the study, told Reuters.
Ordinary matter makes up everything from the cosmic (planets and stars) to the earthly (people and trees.) But it only accounts for about 15% of matter in all of the known universe.
The vast majority of matter is dark – invisible until it is detected only through its gravitational effects.
Unlike dark matter, ordinary matter emits light in various wavelengths, which allow it to easily be seen. Still, scientists have long struggled to account for where all of it is located since a large chunk of ordinary matter is spread so thin among galaxies and the vast spaces between them.
For that reason, about half of ordinary matter has long been considered missing.
Until now.
Powerful bursts of radio waves emanating from 69 locations in the cosmos have helped researchers at long last find the missing matter.
The discovery came from a team of astronomers at the California Institute of Technology and the Center for Astrophysics, a research institute jointly operated by the Harvard College Observatory and Smithsonian Astrophysical Observatory.
The team studied brief, bright radio flashes in the distant cosmos, called fast radio bursts (FRBs), to illuminate the matter lying between the radio waves and Earth.
Astronomers have been studying fast radio bursts from across the universe since 2007 when the first millisecond-long burst was discovered.
The bright burst of electromagnetic radiation may be brief, but fast radio bursts are so powerful that they produce more energy than what our sun emits in an entire year, astronomers say.
The 69 radio frequencies the team studied were located at distances ranging up to about 9.1 billion light-years from Earth – making the furthest one the most distant fast radio burst ever recorded. The previous record was a fast radio burst documented about 8 billion light-years away in 2023.
By measuring how the light from the radio bursts spread and dispersed – not unlike how a prism turns sunlight into a rainbow – while traveling toward Earth, the astronomers were able to determine how much matter was in their path.
"If you see a person in front of you, you can find out a lot about them," Vikram Ravi, a Caltech astronomer who coauthored the study, said in a statement. "But if you just see their shadow, you still know that they're there and roughly how big they are."
The results revealed that about 75% of the universe's ordinary matter resides in the space between galaxies, also known as the intergalactic medium.
How did it all end up in the middle of nowhere? Astronomers theorize it happens as gas is ejected from galaxies when massive stars explode in supernovas, or when supermassive black holes inside galaxies expel material after consuming stars or gas.
The remaining 15% of the "missing" matter exists within either galaxies in the form of stars and cold galactic gas, or in the halos of diffuse material around them, according to the researchers.
While this distribution is in line with predictions from advanced cosmological simulations, this is the first time it has been observed and confirmed, the researchers claim. The findings will help researchers better understand how galaxies grow.
Caltech is also planning for its future deep-space radio telescope in the Nevada desert, the DSA-2000, to build upon the findings when it becomes operational. The radio array is being planned to detect up to 10,000 fast radio bursts per year.
The findings were published June 16 in the journal Nature.
Contributing: Reuters
Eric Lagatta is the Space Connect reporter for the USA TODAY Network. Reach him at [e-mail]
This article originally appeared on USA TODAY: Astronomers just found the universe's 'missing' matter: Here's how