Addressing the Possibilities of What Dark Matter is Made Of
In the annals of science, it will go down as a contest for the ages: the race to discover dark matter. This elusive substance has mystified us since the 1930s, when astronomers first realized galaxies needed some kind of invisible gravitational glue to hold them together. No one knew what it was, so the name dark matter stuck. Beggaring belief, the universe seems to hold more than five times as much dark matter as it does “normal” matter. This means it should literally lurk right under our noses, permeating and penetrating Earth as our solar system swings through our galaxy, which (like most massive galaxies) is brimming with the stuff.
Yet for all that seeming ubiquity, scientists know shockingly little about the universe’s dominant material. Dark matter could be made of one kind of particle or many. Those particles might be massively heavy or wispily light. We think it only interacts with other matter (and itself) via gravity, but dark matter could turn out to have interactions with any force of nature — known or unknown.
Addressing all these possibilities, physicists have conjured up quite the stable of dark matter candidates. And like race horses, these proposed particle types are vying to win what you might call the Dark Matter Derby, competing through theories, experiments and observations.
The current racecourse weaves from the grandest scales of the universe to the tiniest, from galaxies to subatomic particles. To reach the finish line first, researchers are often literally chilling out: Many of the experiments involve supercooling materials such as liquid xenon to subfreezing temperatures, which makes it easier for the materials’ atoms to bump into stray dark matter particles and thus betray the elusive galactic glue’s existence.
The odds-on favorite, called a WIMP (for weakly interacting massive particle), has been a no-show despite intensive search efforts. Meanwhile, a once highly touted competitor called the massive compact halo object, or MACHO — cheekily named in opposition to the WIMP — has fallen out of contention, its very existence debunked. Some newer long shots, meanwhile, are poised to give the dark matter thoroughbreds a run for their money.
The stakes couldn’t be higher: If any candidate makes it to the winner’s circle, it would deliver a sweeping rewrite of how the universe works on the most fundamental levels. At this pivotal stage in the race, here is a breakdown of the Dark Matter Derby’s slate of entrants, from front-runners to (ahem) dark horses. Berkeley Physics Professor and former Director of the KAVLI Institue Hitoshi Murayama weighs in on the top candidates.