For the first time ever, astronomers think they've spotted dark matter interacting with matter outside of the forces of gravity. This is a big deal, because it could help us finally understand the matter that accounts for 85 percent of the Universe's mass.
Dark matter has only ever been detected indirectly using a technique known as gravitational lensing, where astronomers observe the way dark matter bends the light of distant galaxies. And up until now it was always thought that gravity was the only force that affected dark matter at all, leaving it travelling through the Universe pretty much unnoticed, hence the name 'dark'.
But now astronomers report they've seen dark matter "slowing down" after interacting with other dark matter, suggesting it's able to engage with forces other than gravity.
"We used to think that dark matter sits around, minding its own business. But if it slowed down during this collision, this could be the first dynamical evidence that dark matter notices the world around it," the lead researcher, Richard Massey, from Durham University in the UK, explained in a press release.
They made the discovery after observing a deep-space collision between four galaxies, and studying the movement of their surrounding dark matter. Surprisingly, they found one 'clump' of dark matter was lagging behind its associated galaxy by 5,000 light years, or 50,000 million million kilometres.
This kind of lag is predicted to happen if dark matter is interacting, even very slightly, with forces other than gravity, the researchers explain in the Monthly Notices of the Royal Astronomical Society.
"Our observation suggests that dark matter might be able to interact with more forces than just gravity, meaning we could rule out some key theories about what dark matter might be," said team member Liliya Williams, from the University of Minnesota. "The parallel Universe going on around us has just got interesting. The dark sector could contain rich physics and potentially complex behaviour."
Just last month, the team published a study that found dark matter interacted very little during 72 galaxy cluster collisions, which are usually very quick and dynamic. But this new research looked at collisions between individual galaxies instead, which can last much longer.
Taken together, these two studies offer some kind of range that dark matter might interact between.
"We are finally homing in dark matter from above and below - squeezing our knowledge from two directions," said Massey in the release. "Dark matter, we're coming for you."