A new study carried out by the ESO's Very Large Telescope and the NASA/ESA Hubble Space Telescope has revealed for the first time that dark matter may well interact with itself - a discovery that, at first glance, seems to contradict what we thought we knew about the nature of this invisible mass.
So what's going on?
ANALYSIS: Dark Matter Just Got Darker (and Weirder)
Using the advanced MUSE instrument at the VLT and Hubble, astronomers were able to zoom in on 4 colliding galaxies within the galaxy cluster Abell 3827. This cluster is huge and is often the hunting ground for enigmatic gravitational lenses.
All galaxies are comprised primarily of dark matter. Without this invisible mass, which accounts for 85 percent of the mass of the entire universe, observations of the visible matter within a spinning galaxy wouldn't make much sense; the stars should fly apart. But with the presence of dark matter, galaxies have the gravitational bulk to retain their structure.
So when observing Abell 3827, astronomers know that dark matter is there through observations of gravitational lenses - starlight from galaxies behind the cluster becomes warped and bent around curved spacetime. If you remove the gravitational influence of the visible galaxies, a huge gravitational component remains, allowing astronomers to accurately measure the quantities and locations of dark matter clouds within the galaxy cluster.
ANALYSIS: LHC Ready to Hunt Down Mystery Dark Matter Particles
Using this method to map out the distribution of dark matter in 4 colliding galaxies, astronomers have deduced that the dark matter associated with each galaxy is lagging some 5,000 light-years behind the normal matter in those galaxies.
What's causing this lag? According to the study, published in the journal Monthly Notices of the Royal Astronomical Society on April 15, it is being caused by some kind of interaction between the galactic halos of dark matter within the colliding galaxies. As the galaxies collide, the visible matter interacts as expected, but the dark matter halos appear to have a net drag effect on one another, creating the 5,000 light-year lag.
"We used to think that dark matter just sits around, minding its own business, except for its gravitational pull," said Richard Massey of Durham University and lead author of this study. "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."
This is an exciting discovery, but to understand what kind of mutual dark matter interaction is causing this large-scale effect, we need more observations and refined computer simulations.
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"We know that dark matter exists because of the way that it interacts gravitationally, helping to shape the Universe, but we still know embarrassingly little about what dark matter actually is," said co-investigator Liliya Williams of the University of Minnesota. "Our observation suggests that dark matter might interact with forces other than gravity, meaning we could rule out some key theories about what dark matter might be."
This possible dark matter interaction appears to contradict the recent survey of 72 galaxy cluster collisions. The result of that study indicated that dark matter interacts very little (if at all) with itself.
But this new study zooms in on individual colliding galaxies, not entire clusters. According to Massey's team, the collisions studied here focuses on the behaviour of dark matter over galactic scales and the galactic collisions studied here have likely lasted longer than those observed in the previous study, increasing the observed lag effect over time.
Regardless, both studies have provided a fascinating insight to possible dark matter interactions over two different scales.
"We are finally homing in on dark matter from above and below - squeezing our knowledge from two directions," said Massey.