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Two models of dark matter density structure in the early universe. In the cold dark matter model (top), the filament fragments into numerous, nearly spherical halos, leading to large and short-lived stars. In the warm dark matter model (bottom), the moderately energetic nature of the dark matter prevents those type of structures from forming. (Science)

The speed or "temperature" of the mysterious substance known as dark matter could have hada direct impact on how the early universe formed, according to scientists who suggest the invisible substance may be "warmer" or more energetic than previously thought.

Experiments using computer simulations suggest the formation of dark matter just after the big bang some 13 billion years ago played a key role in determining where the first stars would form, said Tom Theuns, an astronomer at Durham University in the U.K., who led the study published in the journal Science on Thursday.

But what kind of stars would form in those early stages of the universe would depend on the nature of dark matter itself, Theuns said.

Little is known about dark matter, since it does not emit or absorb light or other electromagnetic radiation. But astronomers have inferred its existence from its influence on the gravity of visible objects like stars and galaxies, and it's thought to make up more than 80per cent ofmatter in the universe.

Astronomers once suspected the missing mass could have come from speedy or "hot" particles such as neutrinos, which travel at close to the speed of light and could have mass, however small. But the prevailing theory is these particles would be moving too fast to cool enough to clump and form the structures of dark matter astronomers assume are present today.

As a result, most astronomers favour the theory that dark matter is "cold," or made of slow moving particles more likely to clump together as it cools.

Using computer simulations of the cooling process of dark matter expanding out from the big bang, Theuns and Durham University colleague Liang Gao built two models: one using cold dark matter and the other using "warm" dark matter, which it defines as matter that moves fast enough that only larger structures of dark matter would be created.

What they found was that both model created stars, butin slightly different ways.

The first stars were made up of hydrogen and helium, the only elements present in the early universe. In the cold dark matter model, these stars condensed in large clumps of dark matter, creating giant stars 100 times larger than the sun but with short life spans. In the warm dark model, the dark matter was more smoothly distributed, leading to the formation of stars of all sizes, including some smaller ones that might be detectable today.

Finding these ancient stars amid the billions of stars in the Milky Way would be a challenge, said University of Toronto astronomer Michael Kesden.

Astronomers might start the search by looking for stars with only hydrogen and helium present inside them, but that alone may not be proof of a star's age, he told CBCnews.ca.

"It sounds strange to say, but in some ways the internal structures of stars are even less understood than dark matter," he said. "There could be some other stellar process that is getting rid of the metals and other elements in a star's core that we don't know about."

Both Kesden and Marla Geha, a professor at the NRC Herzberg Institute of Astrophysics in Victoria, said the paper proposed interesting ideas, though both scientists work with "cold" dark matter models.

Among the paper's other suggestions are that supermassive black holes, found at the centre of galaxies, are the result of collisions between early stars and "warm" dark matter.