Scientists said they had found tantalising evidence suggesting "dark matter" - the mysterious substance believed to comprise most of the Universe's mass - may be more dynamic than thought.
Theorised by physicists back in the 1930s, dark matter cannot be seen by telescopes, and its existence is inferred indirectly, through its gravitational effects on visible matter.
Calculations based on the Standard Model of cosmology suggest it accounts for nearly 85 percent of the mass in the Universe, stretching out in clumpy tendrils that enfold galaxies.
But until now, that was pretty much all we knew, said Richard Massey, an astrophysicist at Durham University, northeastern England, who led the new study.
"Everything that we've known about dark matter until now is that it sits around doing nothing, affects the Universe around it but doesn't do anything else," he said in an interview with AFP.
"We've found for the first time that it may have more tricks up its sleeve -- it might be affecting things around it in other ways, through other forces."
The evidence comes from a unique chance, using the US Hubble orbital telescope and the European Southern Observatory's Very Large Telescope in Chile.
Together, the two high-powered facilities viewed the simultaneous collision of four distant galaxies at the centre of a galaxy cluster a very distant 1.3 billion light years from Earth.
What grabbed the astronomers' attention was that one dark matter clump seemed to be lagging behind the galaxy it surrounds.
The gap was big -- 5,000 light years, or 50,000 million million kilometres (31 million million miles).
- 'Pea-souper' -
The lag was apparently caused by a "fog" of dark matter and hydrogen atoms, which slowed the advance of the dark-matter clump, said Massey.
"It's like moving through a thick pea-souper, so thick that you get a little bit of friction from it as you walk down the street, and it slows you down," Massey said in a phone interview.
"(The galaxy's) dark matter has taken a different trajectory and ended up in a different place," he said. "This is really amazing, because we don't see this in any other galaxy."
What the interaction is remains unclear -- it could be a well-known force or something exotic, but it is clearly not gravity.
Massey admitted that the discovery only slightly moved the needle when it came to learning more about dark matter.
But it did validate a theoretical pathway about the enigmatic stuff.
"It's embarrassing how little we know about dark matter," he said.
"Until now, we've known almost nothing. As far as we could tell, it didn't interact in any way.
"But there are lots of theoretical models about what dark matter might be and what it could do, and some of them did in fact suggest it would interact in certain ways."
The study appears in the British journal Monthly Notices of the Royal Astronomical Society (RAS).
Liliya Williams, a professor at the University of Minnesota, said that dark matter -- far from being inert -- could turn out to interact with more forces than just gravity.
"The parallel Universe going on around us has just got interesting," Williams said in a press release. "The dark sector could contain rich physics and potentially complex behaviour."