Half the universe's mass has been discovered, ending a decades-long mystery

Astronomers have located the universe’s missing matter.

Ordinary matter includes all matter made of atoms — such as stars, planets, gas, and dust. It accounts for about 15 per cent of all the matter in the universe, with the rest made up of dark matter.

For decades, scientists struggled to account for all the ordinary matter predicted by cosmological models. Observations of visible stars, galaxies, and gas clouds revealed only about half of the expected number of ordinary matter. So, where did it all go?

They may now have an answer.

A team of physicists led by Boryana Hadzhiyska from the University of California, Berkeley, found that the matter is there, in the form of diffuse ionized hydrogen. "We think that, once we go farther away from the galaxy, we recover all of the missing gas," said Hadzhiyska.

Scientists have found the universe's missing matter Photo by Jeremy Perkins on Unsplash

"To be more accurate, we have to do a careful analysis with simulations, which we haven't done. We want to do a careful job."

Ionized hydrogen can’t be seen with regular telescopes, so scientists turned to a method that uses the oldest light in the universe — the cosmic microwave background (CMB). By examining an effect known as the kinematic Sunyaev-Zel’dovich (kSZ) effect, they were able to detect subtle distortions in the CMB caused by this diffuse gas

The method involved stacking images of millions of galaxies and comparing them with precise measurements of the CMB to reveal the presence of the hidden matter.

“The cosmic microwave background is in the back of everything we see in the universe. It’s the edge of the observable universe,” explained Simone Ferraro, a senior scientist at Lawrence Berkeley National Laboratory and UC Berkeley.

The team essentially used the CMB as a backlight and measure how it changed as it passed through clouds of ionized gas. They found that the gas was much more spread out and fainter than they had thought — extending up to five times farther than astronomers had previously assumed.

The results are being peer reviewed by Physical Review letters.

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