By looking at four colliding galaxies at the centre of cluster Abell 3827, located 1.3 billion light years away from Earth, an international group of scientists, led by researchers at Durham University, UK, may have found the first signs that dark matter is not as “dark” as previously thought.
Using the MUSE instrument on ESO’s Very Large Telescope in Chile, along with images from Hubble in orbit, the team has found that one clump of dark matter appeared to be lagging behind the galaxy it surrounds, indicating for the first time that this mysterious substance interacts with forces other than gravity.
The lagging clump, offset from its galaxy by 5.000 light years, was identified via the gravitational lensing effect. As the collision was taking place in front of an unrelated spiral galaxy, the mass of dark matter surrounding the four colliding galaxies distorted spacetime, thereby twisting the image of the background galaxy into characteristic arc shapes.
Our current understanding is that all galaxies exist in clumps of dark matter (comprising around 85% of all the matter in the universe) which holds them together with its gravity.
Never before had this invisible type of matter been observed to interact in any way other through the force of gravity.
“We used to think that dark matter just sits around, minding its own business, except for its gravitational pull. But if dark matter were being slowed down during this collision, it could be the first evidence for rich physics in the dark sector – the hidden Universe all around us,” said team member Liliya Williams from the University of Minnesota.
The researchers note that while their findings are extremely intriguing, more work has to be done yet to rule out other possible effects that might cause the observed lag between dark matter and the galaxy it hosts. Similar observations and collision simulations are already under way.
This study follows from previous research the team did which observed 72 collisions between galaxy clusters and found that dark matter interacts very little with itself. But as the present effort focused on collisions between individual galaxies – which could have lasted longer – the researchers have been able pick up on the subtle effects of friction that have built up over time.
Looked at side by side, these two papers bracket the behaviour of dark matter for the first time: “It’s just really exciting, because for about 10 years now, the news has all been about how little dark matter interacts. It seems to do less and less,” said study lead author Richard Massey from the Durham University’s Institute for Computation Cosmology. “But finally, dark matter seems to be doing something! It seems to care about the world around it.”