Astronomers using a powerful quasar to study an enormous invisible tendril full of superheated gas say they may have finally discovered the universe’s “missing” detectable matter.

The findings, published in the journal Nature, solve a decades-old mystery and could help scientists further probe the structure and evolution of the cosmos.

All of the atoms in the stars, galaxies and planets in existence make up roughly 5 percent of the mass-energy density of the cosmos. The majority, about 70 percent, is made up of dark energy — a mysterious, repulsive force that is causing the universe to expand. The remaining quarter or so is made up of dark matter — invisible stuff that can only be felt by its gravitational influence on galactic scales. Dark matter connects clusters of galaxies with massive tendrils, forming a cosmic web that serves as an unseen skeleton for the universe.

That small slice of normal matter that we can directly detect, which scientists call baryonic matter, is the most known quantity of the three: It emits light (like the sun) or reflects it (like the moon), making it visible to us or detectable by telescopes. And yet it also presents its own mystery, because for decades, scientists haven’t been able to find all of it.

“Over 20 years ago people noted that if you added up all the starlight and all the mass in galaxies that goes with that starlight, you only get about 10 percent of that 5 percent of ordinary matter,” said co-author J. Michael Shull, an astrophysicist. “So there was a ‘missing matter’ problem going back over 20 years: where is the gas, where are the baryons, that aren’t collapsed into stars and galaxies? … It really goes to the heart of key predictions in cosmology about the big bang.”

The missing matter would be mostly made out of hydrogen, the simplest element and the most abundant in the universe. If a cloud of ionized hydrogen sits between Earth and a source of ultraviolet light, that hydrogen will leave a distinct chemical fingerprint.

The problem is that as the gas gets hotter and hotter — say, above a million degrees Kelvin — ionized hydrogen stops leaving a clear signal in ultraviolet. So researchers had to target much rarer oxygen ions.

The scientists used a European Space Agency’s space telescope to study the BL Lacertae quasar 1ES 1553+113, a supermassive black hole at the center of a galaxy. Quasars gobble up matter and shine brightly in many wavelengths of light. Studying the chemical fingerprint of oxygen in the X-rays from the quasar light, the scientists found a large amount of extremely hot intergalactic gas — so much that they calculate that this gas could account for up to 40 percent of the baryonic matter in the cosmos, enough to explain the missing matter.

The researchers think that these ions may have started in the hearts of stars that went supernova, and were thrown out of their home galaxies by these explosive stellar deaths.