A massive instrument circling the globe aboard the International Space Station has provided a first glimpse of what may be mysterious dark matter in the universe, experts said Wednesday.

The elusive nature of dark matter -- which surrounds the galaxy in a near-spherical halo and is believed to make up around a quarter of the universe -- is considered one of the most important riddles in physics.Dark matter has only been observed indirectly through its interaction with visible matter, and is not explained by the standard model of physics.But that could change with data collected by the Alpha Magnetic Spectrometer, the most sensitive particle physics spectrometer ever sent to space, made by a 16-nation team at a cost of $2.5 billion.

The machine arrived at the orbiting outpost aboard the US space shuttle Endeavour's last flight in 2011.The first results from the AMS seem to indicate "evidence of a new physics phenomena," said a press statement from the international research team.The AMS studies cosmic rays -- charged high-energy particles that permeate space -- before they interact with Earth's atmosphere.Of the 25 billion cosmic ray events the AMS has studied so far, "an unprecedented number, 6.8 million, were unambiguously identified as electrons and their antimatter counterpart, positrons," said CERN, the European Organization for Nuclear Research.

AMS spokesman Samuel Ting said more experiments in the coming months should indicate whether these positrons are a signal for dark matter or not.There is still a chance that the signals could result from pulsars, or neutron stars that emit electromagnetic radiation."There is no question we are going to solve this problem," Ting told reporters.Scientists are pinning their hopes on the AMS because its accuracy is "unmatched by any other experiment," said Ting, a Nobel laureate and professor at the Massachusetts Institute of Technology (MIT).

"Our evidence supports the existence of dark matter but cannot rule out" other origins, such as pulsars, he said, stressing that scientists are not yet convinced either way, but are awaiting more data.The findings appear in the journal Physical Review Letters.Michael Salamon, US Department of Energy Office of Science program manager for AMS, described the latest measurements as "exciting," but preliminary.It is very important to say there may be a very common, prosaic explanation for this excess. It could be due to pulsars, for example. We don't know," he told reporters.

"The fact is AMS has made a high-precision measurement and in the future, with more statistics, we are going to learn more about the nature of this excess. And if nature is kind, we might have a very exciting discovery in the future," he added.The AMS is expected to keep operating until 2020, or as long as the ISS stays in operation."If we detect dark matter and learn something about its nature we will have made a major impact to our understanding of physics and nature itself," Salamon said.