On September 14, 2015, at 3:50 AM Central time, a tiny vibration shuddered down the 2.5-mile-long arms of a massive machine in Livingston, Louisiana. A fraction of a second later, a similar vibration shook the arms of an identical machine in Hanford, Washington. Eventually, physicists from those facilities confirmed the nature of those twinned tremors: After a century of work, they’d finally seen gravitational waves. That tiny vibration, they found, originated from a cataclysmic collision between two black holes, 1.5 billion years ago.

Just two months later, the Laser Interferometer Gravitational-Wave Observatory detected a second wave. Then, this year, a third—then a fourth, fifth, and sixth. Together, they have solved a long-standing mystery in physics, confirming that gravity obeys Einstein’s theory of general relativity. And in October, three pioneering gravitational wave researchers—Barry Barish, Kip Thorne, and Rainer Weiss—won the Nobel Prize for that critical work.

But don’t let the accolades make you think that LIGO’s success came easy. When the observatory started laying crucial infrastructure more than 20 years ago, the rest of the field viewed them as outsiders; a colleague even testified against congressional funding for their efforts. And LIGO isn’t an outlier; all big discoveries in physics inevitably follow a meandering (and costly) path of scientific labor and political sparring. The next discovery will again depend on a capricious combination of hard work, politics, and luck—so physicists have no idea which breakthrough will come next.

But you can expect it to be expensive. Physicists have solved many of the simpler mysteries in the universe, says astrophysicist Joshua Frieman of the University of Chicago, and the remaining questions are complicated enough to require multi-million, custom-made facilities. “We’re victims of our own success,” he says.

So what might be that next great existential breakthrough in physics? Some items on the docket: discover dark matter and dark energy. And oh yeah, figure out where the matter in the universe comes from.

“I think people think of scientists only in the moment of discovery,” says physicist Luca Grandi of the University of Chicago—a dark matter hunter whose search began in 1999. As a college student in Italy, he joined a dark matter collaboration called WArP, whose own mission was already six decades in the making. In 1933, Fritz Zwicky first predicted the existence of invisible “dark matter” when he noticed galaxies were spinning faster than their masses predicted. Decades later, Vera Rubin found more evidence of dark matter in other galaxies. Physicists now think dark matter makes up 85 percent of the universe’s mass.

Still, no one has seen the stuff down on Earth. In his 18 years in the biz, Grandi has tried several tactics. In 2008, as a postdoctoral researcher in the US, he co-founded his own dark matter collaboration called DarkSide, which is still ongoing. But DarkSide’s argon-based detector fell out of favor, as the community pivoted to xenon-based detectors, which were more precise.

Grandi’s current dark matter group, aptly named Xenon1T, is already making plans into the 2030s—first expanding its current 3-ton detector into 8 tons, and then eventually 50 tons. The bigger the detector, the more likely it’ll catch a weakly interacting massive particle, or WIMP, a hypothesized dark matter particle. And Grandi, now 41, is willing to stay the course. “Every day is different, so I think it’s difficult to get tired in this field,” he says. “You really do a lot of things, from hardware to data analysis to interpretation and statistics. So it’s always exciting.”