What Astronomers Just Discovered Between Galaxies Changes Everything

For decades, scientists have known that a massive chunk of the universe’s ordinary matter was missing. Not dark matter, the elusive substance that doesn’t interact with light, but regular, everyday matter made of atoms. 

And now, thanks to a brilliant use of cosmic radio signals, that mystery may finally be solved.

In a new study published in Nature Astronomy, astronomers used fast radio bursts (FRBs)—brief, millisecond-long blasts of energy from deep space—to detect where all that missing matter was hiding: in the vast stretches between galaxies, known as the intergalactic medium.

These FRBs are powerful. Though short-lived, they emit as much energy in one burst as the sun does in 30 years. When they pass through space, they act like cosmic flashlights, lighting up the otherwise invisible gas that floats between galaxies. 

The team measured how the light from 69 FRBs slowed as it moved through this matter, allowing them to "weigh" the fog they passed through.

"It’s like we're seeing the shadow of all the baryons," explained Caltech assistant professor Vikram Ravi, using the scientific term for this ordinary matter. "With FRBs as the backlight, we now know roughly where the rest of the matter in the universe is hiding."

The results show about 76 percent of the universe’s baryonic matter exists in this intergalactic fog. Meanwhile, 15 percent of the baryonic matter surrounds galaxies in halos and just 9 percent resides inside the galaxies themselves.

This breakthrough was made possible by telescopes like Caltech’s Deep Synoptic Array and Australia’s Square Kilometre Array Pathfinder, which helped localize the FRBs' origins. Caltech's upcoming DSA-2000 radio telescope, set to detect 10,000 FRBs per year, could be the key to even deeper cosmological insights.

For astronomers, it's a milestone moment—one that brings us closer to understanding not just where we come from, but how the universe is truly structured.