One of the biggest mysteries in science today is what makes up dark matter—the plentiful, invisible material that swarms throughout the universe, exerting its gravitational lure on regular matter. Physicists have traditionally surmised that dark matter is a single type of particle that rarely interacts with the rest of the particles in nature, such as the normal electrons and quarks that make up the atoms in our bodies. But that picture is not the only option.

Lately another theory, sometimes called complex dark matter, has been gaining prominence. This scenario posits not just one category of particle composing dark matter but many—and it includes the possibility that some of these particles could combine to form composites akin to dark atoms. Complex dark matter could act and group in different ways than the simple dark matter particle is thought to do. It could, for example, form structures that mirror and overlap with the disks of spiral arms in galaxies.

Physicists Bogdan Dobrescu and Don Lincoln, both of the Fermi National Accelerator Laboratory in Illinois, describe the possibilities of complex dark matter in the latest issue of Scientific American. Read their article here. Lincoln also details the idea in a recent video, below: