Just 13 percent of the universe is thought to be composed of "normal" matter, according to a recent study. Normal matter is anything made of baryons, subatomic particles that include neutrons and protons -- the stuff of plants, rocks and people.

The balance of material that must be out there is called "dark matter." Some of this is simply normal matter that can't be seen -- cold, dead stars and small rocks, for example. But the bulk of dark matter is thought to involve mysterious, unseen particles.

Researchers say dark matter is needed to explain why galaxies don't fly apart.

In the new study, astronomers spent two decades measuring helium-3, a rare isotope of helium that is thought to be a primordial element -- one that has been around since the universe began. Among their results was the discovery that helium-3 does not seem to be created or destroyed during star formation, so the amount they measured in our galaxy is likely representative of how much was present just moments after the Big Bang.

The results were presented in the Jan. 3 issue of the journal Nature.

"Moments after the Big Bang, protons and neutrons began to combine to form helium-3 and other basic elements," said Robert Rood of the University of Virginia. "By accurately measuring the abundance of this primordial element in our galaxy today, we were able infer just how much matter was created when the universe was only a few minutes old."

Rood and his colleagues, Thomas Bania from Boston University and Dana Balser from the National Radio Astronomy Observatory, conducted their study using the National Science Foundation's 140-foot radio telescope in Green Bank, West Virginia.

The researchers looked throughout the Milky Way Galaxy for helium-3. No matter where they looked -- in areas of intense star formation or in quieter regions -- the relative abundance of helium-3 remained constant.

"Since helium-3 has not been created or destroyed in our Galaxy in any appreciable amounts, then what we detected is most likely equal to the abundance of primordial helium-3 created by the Big Bang," Bania said.

Bania added that the finding will force astronomers to rethink models of star formation and even the inner workings of the Sun.

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