For years, dark matter has been behaving badly.  The term was first invoked nearly 80 years ago by the astronomer Fritz Zwicky, who realized that some unseen gravitational force was needed to stop individual galaxies from escaping giant galaxy clusters.  Later, Vera Rubin and Kent Ford used unseen dark matter to explain why galaxies themselves don’t fly apart. 

Yet even though we use the term “dark matter” to describe these two situations, it’s not clear that the same kind of stuff is at work.  The simplest and most popular model holds that dark matter is made of weakly interacting particles that move about slowly under the force of gravity.  This so-called “cold” dark matter accurately describes large-scale structures like galaxy clusters.  However, it doesn’t do a great job at predicting the rotation curves of individual galaxies.  Dark matter seems to act differently at this scale. 

In the latest effort to resolve this conundrum, two physicists have proposed that dark matter is capable of changing phases at different size scales.  Justin Khoury, a physicist at the University of Pennsylvania, and his former postdoc Lasha Berezhiani, who is now at Princeton University, say that in the cold, dense environment of the galactic halo, dark matter condenses into a superfluid — an exotic quantum state of matter that has zero viscosity.  If dark matter forms a superfluid at the galactic scale, it could give rise to a new force that would account for the observations that don’t fit the cold dark matter model.  Yet at the scale of galaxy clusters, the special conditions required for a superfluid state to form don’t exist; here, dark matter behaves like conventional cold dark matter. 

“It’s a neat idea,” said Tim Tait, a particle physicist at the University of California, Irvine. “ You get to have two different kinds of dark matter described by one thing.”  And that neat idea may soon be testable.  Although other physicists have toyed with similar ideas, Khoury and Berezhiani are nearing the point where they can extract testable predictions that would allow astronomers to explore whether our galaxy is swimming in a superfluid sea. 

Impossible Superfluids

Here on Earth, superfluids aren’t exactly commonplace.  But physicists have been cooking them up in their labs since 1938.  Cool down particles to sufficiently low temperatures and their quantum nature will start to emerge.  Their matter waves will spread out and overlap with one another, eventually coordinating themselves to behave as if they were one big “superatom.”  They will become coherent, much like the light particles in a laser all have the same energy and vibrate as one.  These days even undergraduates create so-called Bose-Einstein condensates (BECs) in the lab, many of which can be classified as superfluids. 

Superfluids don’t exist in the everyday world — it’s too warm for the necessary quantum effects to hold sway.  Because of that, “probably ten years ago, people would have balked at this idea and just said ‘this is impossible,’” said Tait.  But recently, more physicists have warmed to the possibility of superfluid phases forming naturally in the extreme conditions of space.  Superfluids may exist inside neutron stars, and some researchers have speculated that space-time itself may be a superfluid.  So why shouldn’t dark matter have a superfluid phase, too? 

To make a superfluid out of a collection of particles, you need to do two things: Pack the particles together at very high densities and cool them down to extremely low temperatures.  In the lab, physicists (or undergraduates) confine the particles in an electromagnetic trap, then zap them with lasers to remove the kinetic energy and lower the temperature to just above absolute zero.