Back in the 1990s, experiments began trying to detect the particles that make up dark matter, the ubiquitous yet untouchable invisible material that apparently fills the cosmos. Since then, physicists have found more and more evidence that dark matter is real but not a single sign of the stuff itself. A new version of the long-running XENON experiment that started up late last year aims to finally break that pattern.

Some scientists are excited about the idea that dark matter may be a composite particle—a conglomerate of “dark quarks” and “dark gluons” that stick together just like regular quarks and gluons to create “dark nuclei.” It is also possible that dark matter is not a particle at all. One idea still in the running is that the missing matter is made of primordial black holes that formed soon after the big bang...

Scientific American: Dark Matter’s Last Stand