It makes up about 85 percentof the total mass of the Universe, and yet, physicists still have no idea what dark matter actually is. But a new hypothesis might have gotten us closer to figuring out its identity, because physicists now suspect that dark matter has been changing forms this whole time - from ghostly particles in the Universe's biggest structures, to a strange, super-fluid state at smaller scales. And we might soon have the tools to confirm it. Dark matter is a hypothetical substance that was proposed almost a century ago to account for the clear imbalance between the amount of matter in the Universe, and the amount of gravity that holds our galaxies together.
We can't directly detect dark matter, but we can see its effects on everything around us - the way galaxies rotate and the way light bends as it travels through the Universe suggests there's far more at play than we're able to pick up. And now two physicists propose that dark matter has been changing the rules this whole time, and that could explain why it's been so elusive.
"It's a neat idea," particle physicist Tim Tait from the University of California, Irvine, who wasn't involved in the study, told Quanta Magazine. You get to have two different kinds of dark matter described by one thing."
The traditional view of dark matter is that it's made up of weakly interacting particles such as axions, which are influenced by the force of gravity in ways that we can observe at large scales. This 'cold' form of dark matter can be used to predict how massive clusters of galaxies will behave, and fits into what we know about the 'cosmic web' of the Universe - scientists suggest that all galaxies are connected within a vast inter galactic web made up of invisible filaments of dark matter. But when we scale down to individual galaxies and the way their stars rotate in relation to the galactic center, something just doesn't add up.
"Most of the mass [in the Universe], which is dark matter, is segregated from where most of the ordinary matter lies," University of Pennsylvania physicist Justin Khoury explains in a press statement. On a cosmic web scale, this does well in fitting with the observations. On a galaxy cluster scale, it also does pretty well. However, when on the scale of galaxies, it does not fit."
Khoury and his colleague Lasha Berezhiani, now at Princeton University, suggest that the reason we can't reconcile dark matter's behavior on both large and small scales in the Universe is because it can shift forms. We've got the 'cold' dark matter particles for the massive galaxy clusters, but on a singular galactic scale, they suggest that dark matter takes on a super-fluid state. Super-fluids are a form of cold, densely packed matter that has zero friction and viscosity, and can sometimes become a Bose-Einstein condensate, referred to as the 'fifth state of matter'. And as strange as they sound, super-fluids are starting to appear more accessible than ever before, with researchers announcing just last month that they were able to create light thatacts like a liquid - a form of super-fluid - at room temperature for the first time. The more we come to understand super-fluids, the more physicists are willing to entertain the idea that they could be far more common in the Universe than we thought.
"Recently, more physicists have warmed to the possibility of super-fluid phases forming naturally in the extreme conditions of space," Jennifer Ouellette explainsfor Quanta Magazine. Super-fluids may exist inside neutron stars, and some researchers have speculated that space-time itself may be a super-fluid. So why shouldn't dark matter have a super-fluid phase, too?"
The idea is that the 'halos' of dark matter that exist around individual galaxies create the conditions necessary to form a super-fluid - the gravitational pull of the galaxy ensures that it's densely packed, and the coldness of space keeps the temperature suitably low. Zoom out to a larger scale, and this gravitational pull becomes too weak to form a super-fluid. The key here is that the existence of super-fluid dark matter could explain the strange behaviors of individual galaxies that gravity alone can't explain - it could be creating a second, as-yet-undefined force that acts just like gravity within the dark matter halos surrounding them. As Ouellette explains, when you disturb an electric field, you get radio waves, and when you disturb a gravitational field, you get gravitational waves. When you disturb a super-fluid? You get phonons (sound waves), and this extra force could work in addition to gravity.
"It's nice because you have an additional force on top of gravity, but it really is intrinsically linked to dark matter," Khoury told her. "It's a property of the dark matter medium that gives rise to this force."
We should be clear that this hypothesis is yet to be peer-reviewed, so this is all squarely in the realm of the hypothetical for now. But it's been published on the pre-print website arXiv.org for researchers in the field to pick over. A big thing it has going for it is the fact that it could also explain 'modified Newtonian dynamics' (MOND) - a theory that says a modification of Newton's laws is needed to account for specific properties that have been observed within galaxies.
"In galaxies, there is super-fluid movement of dark matter and MOND applies. However, in galaxy clusters, there is no super-fluid movement of dark matter and MOND does not apply,” the team suggests in a press statement.
We'll have to wait and see where this hypothesis goes, but the Khoury and Berezhiani say they're close to coming up with actual, testable ways that we can confirm their predictions based on super-fluid dark matter. And if their predictions bear out - we might finally be onto something when it comes to this massive cosmic mystery. The research is available online at arXiv.org.
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