“95% of the universe is dark, is invisible to the eye,” according to Pran Nath, Matthews Distinguished University Professor of physics at Northeastern University. This bleak fact highlights the huge mystery of dark matter a light-absorbing, light-emitting substance that constitutes the bulk of the universe’s matter. Physicists have wrestled with its evasiveness for decades, using only gravitational clues to infer its existence. But with experimental hunts repeatedly turning out to be dry holes, scientists are beginning to turn their thoughts toward some bold new ideas that can redefine the universe in our imagination.
The idea that dark matter could consist of a solitary particle, such as long-proposed WIMPs (Weakly Interacting Massive Particles), has been the mainstream science of the last several years. But the lack of finding any solid evidence has been causing increasingly widespread skepticism. “In some ways, I grew tired of WIMPs long ago,” says University of Washington physicist Gray Rybka. That irritation has spawned a move to newer ideas, such as the tantalizing idea of a hidden sector, where dark matter exists in a shadow universe of its own, with distinctive particles and forces.
Hidden-sector theories suggest dark matter particles could interact not through the known Standard Model forces of electromagnetism or the weak force but through completely new dynamics. The particles may even be bound into dark atoms by a dark equivalent of electromagnetism through hypothesized dark photons. Unlike our photons, dark photons may have mass, with the possibility of creating heavy dark atomic nuclei or “nuggets.” The nuggets would drastically affect galaxy evolution, with the potential for supermassive black holes at galaxy centers.
Caltech theoretical physics professor Kathryn Zurek paints this picture: “When you look beyond WIMPs and axions, a whole range of observational consequences open up.” Zurek’s research has been key to studying lighter dark matter candidates, from one-trillionth of the proton mass to one proton. These dark valley particles, as originally characterized, reside in a lower-energy realm that may have flown under the radar in earlier experiments. “We are moving to a new frontier of lighter dark matter,” Zurek says.
This frontier has inspired innovative experimental approaches. Zurek and her collaborators have proposed using phonons quantum oscillations in materials to detect disturbances caused by particles in the hidden sector. Likewise, experiments such as SPICE (Sub-eV Polar Interactions Cryogenic Experiment) and HeRaLD (Helium and Roton Liquid Detector) attempt to exploit materials such as sapphire and superfluid helium in an effort to find subtle interactions. It is only one component of a broader trend in the discipline, as scientists are endeavoring to exploit condensed matter physics to pick up on the faint whispers of dark matter.
The consequences of these ideas stretch far beyond the test tube. Astrophysicists such as Caltech’s Phil Hopkins use computer simulations that include hidden-sector physics to make predictions about how dark matter self-interactions would structure galaxies. “You can imagine a whole dark universe or this hidden sector where all sorts of things are happening underneath normal matter or ‘under the hood,’ as you might say,” Hopkins describes. His estimates indicate that self-interacting dark matter might affect the motion and form of galaxies that can be seen by future telescopes such as the Vera C. Rubin Observatory.
Even the birth of the universe could be hiding dark matter’s secrets. Nath and his team have come up with models that propose the hidden and visible sectors co-evolved in the milliseconds following the Big Bang, contrary to speculation that the two domains evolved separately. Such an interaction would have far-reaching implications for the evolution of the universe, possibly explaining anomalies that baffle the Standard Model.
Even these advances, however, are beset by challenge. The SuperCDMS (Super Cryogenic Dark Matter Search), for example, deep beneath the ground for protection from cosmic rays, is still ramping up sensitivity. Others, such as Caltech’s Sunil Golwala, are eyeing tabletop experiments with which to illuminate hidden-sector particles. “Kathryn is a theorist, and I am an experimentalist. She says, ‘This is the thing you should build in order to look for this model of dark matter.’ I take her ideas and say, ‘OK, we are going to build an experiment that can do that,” Golwala remembers.
And so physicists strive against the limit of their conception and of what their technology permits, and then are humbled by the grandness of their quest. “I am humbled by the universe. We should be embarrassed at some level about how little we know, but this can also be an opportunity to learn more,” says Mark B. Wise, John A. McCone Professor of High Energy Physics at Caltech. His words capture the essence of the dark matter search a quest of curiosity, determination, and hope that knowledge of the unseen forces shaping our universe is within our grasp.
