Scientists have detected particles that may come from invisible "dark matter".
This is thought to make up 23% of the Universe, but can only be detected through its effects on "normal" matter.
Writing in the journal Nature, scientists relate how a satellite-borne instrument found an unexplained source of positrons in space.
But the researchers say their mysterious signal must be further investigated before they will know if they have "discovered dark matter".
The space-based experiment, known as Pamela, was launched in June 2006, and carries instruments designed to investigate dark matter particles.
It detected an "excess" of positrons in an area where the scientists expected to see far fewer.
The instrument that recorded this signal was measuring the ratio of positrons - the anti-particles of electrons that have a positive rather than a negative charge - to electrons.
It found a relatively high ratio of positrons within a "high energy level".
"The ratio [of positrons to electrons] should decrease with increasing energy," said Piergiorgio Picozza, a professor at the University of Rome Tor Vergata, Italy, and one of the authors of the research.
"But we found that, at one particular energy range, it increases, and that's not what we expected."
"Since we know a lot about the background sources of positrons and electrons, we know what fraction we would expect to see at this particular energy level," explained Nigel Smith, who carries out research into dark matter at the UK's Rutherford Appleton Laboratory but was not involved in this research.
"It's like light; when you see different colours, you're looking at photons - light particles - with different energies."
However, there could be another explanation for the positrons. They may come from pulsars - rapidly rotating, super-dense, dead stars that release lots of energy into the cosmos.
Nasa's Fermi Gamma-ray Space Telescope, which was launched in June 2008, is already taking measurements from pulsars and should produce data that could clarify the mysterious signal.
Professor Smith thinks pulsars provide the most likely explanation.
"It's the simplest solution," he said. "I think everyone will be waiting for the Fermi data to come in."
Professor Picozza agrees that pulsars offer a plausible, if less exciting, origin.
"Many leading theoreticians think this signal must come from dark matter," he continued.
"But I don't think this data alone is enough to claim that discovery. What we have found is another primary source of positrons."
He believes that the particles thought to constitute dark matter could be reproduced in the Large Hadron Collider (LHC) in Switzerland.
If, he said, this new signal does not match the data collected from pulsars, and if experiments in the LHC back-up the dark matter theoreticians, "then we could then say we have discovered dark matter".