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Our first glimpse at the universe鈥檚 earliest stars may help us figure out what makes up dark matter. New observations have revealed hydrogen gas in the era when stars were just beginning to turn on, and the gas appears to have interacted with dark matter particles.

After the big bang but before stars and galaxies formed, most of space was filled with hydrogen gas that blended in with the background light remaining from the big bang 鈥� called the cosmic microwave background, or CMB 鈥� rendering it invisible to telescopes.

But as stars began to form, their ultraviolet radiation imparted a bit of energy to the gas atoms, making them able to absorb some of the background light. Astrophysicists have been trying to observe that absorption for decades.

Now, at Arizona State University in Tempe and his colleagues have finally spotted it. They built a specialised radio telescope in the Australian outback, where it can avoid contamination by Earthly radio waves. The frequency of the signal from the early universe overlaps with FM radio frequencies, so the location had to be remote.

Cold and old

Radio spectra of the entire sky showed that hydrogen atoms began absorbing background light about 180 million years after the big bang. 鈥淎side from the CMB, this is the farthest we鈥檝e seen in the early universe,鈥� Bowman says. 鈥淚t鈥檚 the earliest evidence of stars existing.鈥�

Surprisingly, the signal was much stronger than they expected. The strength of absorption depends on the temperature of the gas, so this means that the gas was twice as cold as previously predicted 鈥� about 5 kelvin, or -268掳C. This is far colder than decades of models predicted.

鈥淭he only way to cool something is to transfer energy from it, but what could possibly be even colder than the gas? The only candidate is dark matter,鈥� says at Tel Aviv University in Israel. Everything else in the early universe is just too warm.

Barkana found that the dark matter particles must have been fairly light to absorb enough velocity and thus heat from the gas to account for the drastic drop in temperature. That鈥檚 because it鈥檚 easier to transfer velocity to a lighter particle.

Light dark matter

He calculated that the particles can be no heavier than about 4.3 billion electron volts (GeV), or nearly 4.5 times the mass of a hydrogen atom. That鈥檚 much lighter than the mass of about 100 GeV that we expect for the leading contender for dark matter, called weakly interacting massive particles (WIMPs).

This measurement is a key addition to the evidence we have for what dark matter is and how it looks. 鈥淯ntil now, dark matter has been implied from its gravity. It鈥檚 been indirect, and some people have just questioned our theories of gravity,鈥� says Barkana. 鈥淭his is the first evidence that鈥檚 independent of gravity.鈥�

The researchers remain cautious about their finding, even after two years of double-checking every part of their radio telescope, and building a second exact copy to confirm their measurement. Other experiments will have to independently verify the signal that Bowman and his team found.

鈥淚f the report is correct and if it鈥檚 confirmed, then the implication is going to be a new understanding of dark matter 鈥� how it affected the early universe and the universe now,鈥� says at the Harvard-Smithsonian Center for Astrophysics. 鈥淚t鈥檚 going to change everything.鈥�

Read more: Dark matter may be the source of antimatter streaming past Earth