Eighty years after the discovery of dark matter, physicists remain totally stumped about the nature of this nonreflective stuff that, judging by its gravitational effects, pervades the cosmos in far greater abundance than all the matter we can see.
Rather, there might exist a whole family of dark particles that interact with one another via unknown dark forces, much as ordinary matter consists of quarks, electrons and a bustling zoo of other light-sensitive quanta.
Whereas dark matter of a single, inert type such as an axion would enshroud galaxies in spherical clouds called halos, particles in a dark sector might interact with one another in ways that release energy, enabling them to cool and settle into a lower-energy configuration.
Namely, these cooling dark matter particles would collapse into rotating disks, just as stars and gas settle into pancake-shaped rotating galaxies and planets orbit their stars in a plane.
As Randall discusses in her 2015 book, Dark Matter and the Dinosaurs, the subtle periodicity might happen because space objects get destabilized each time our solar system passes through the dark disk while bobbing up and down on its way around the galaxy.
Still, the presence of any dark disk at all, even one made of a minority of dark matter particles, would revolutionize physics.
Even if there’s no dark disk, it’s still possible that there might be a dark sector.