Scientists have found the first potential evidence of the existence of dark matter.
Dark matter is form of sub-atomic particle which may make up more than a quarter of the universe's energy density - but has never been seen.
Theories of its existence date back to the 1970s, when Vera Rubin noticed that stars near the edge of the galaxy move quicker than predicted. The presence of large amounts of hidden matter across the universe would reconcile those observations with the laws of gravity.
Since then, astrophysicists have been working on experiments to demonstrate the existence of dark matter, despite its elusive nature.
New evidence from the Super Cryogenic Dark Matter Search (SuperCDMS), built a half-mile underground at the Soudan mine in northern Minnesota, might just be the first hints that search is coming to fruition.
According to Texas A&M university, whose physicist Rupak Mahapatra works on SuperCDMS, the experiment has detected a weakly-interacting massive particle, or WIMP, which are thought to make up dark matter.
But the university has warned that the results are a so-called "three-sigma" finding - meaning that it found three signals which only have a 0.19% chance of being found if no particle had created them.
The same instrument reported two potential sightings of dark matter in 2010, but those turned out to just be mistakes made by the highly technical machines involved.
"This is certainly very exciting," Mahapatra said. "But not fully convincing by the standards. We just need more data to be sure. For now, we have to live with this tantalizing hint of one of the biggest puzzles of our time."
The discovery was made by looking for the trails of energy created when WIMPs bounce off atomic nuclei. Other experiments such as the Large Hadron Collider are also set up to look for similar evidence.
But for now the researchers have to go back to their data - they're not sure enough to know that what they've found is dark matter or something even more strange.
"We are only 99.8 percent sure, and we want to be 99.9999 percent sure," said Mahapatra.