The Dark Horse in the Search for Dark Matter

China is proving to be a formidable participant in the hunt for elusive, invisible particles in the universe.

Soldiers in silhouette guard a space launch site.
Chinese soldiers guard the perimeter of the Jiuquan Satellite Launch Center on the eve of the Shenzou 9 launch in 2012. (STR / AFP / Getty)

Two years ago, China launched a space probe into orbit around Earth. Scientists nicknamed it Wukong, or Monkey King, after the hero of a 16th-century novel about a Buddhist monk’s long journey to India to secure religious texts. The probe’s job was to track and record cosmic rays, the streams of high-energy particles that constantly bombard Earth’s atmosphere from all corners of the universe.

In its first 530 days of operation, the probe recorded more than 2.8 billion cosmic rays. When scientists looked at data, they found something unusual. Some of the cosmic rays—at least 1.5 million of them—were recorded at a different and higher energy level than the others. Plotted on a chart, they appeared as a cluster of tiny outliers suspended above the curve.

Though they don’t look like much, this blip is incredibly important to astrophysicists around the world who are trying to solve one of the biggest mysteries in science: the existence of dark matter. Scientists believe that dark matter makes up about a quarter of everything in the universe, but the tricky thing is that we can’t see dark matter. In fact, we don’t even know what it’s made of. The​ existence of dark matter ​is inferred in​directly from observational data. It shapes some important phenomena in the cosmos—like why galaxies form in the first place, and stick together instead of flying apart—that the physics we already understand cannot.

The top theory for dark matter suggests that the mysterious stuff is made of WIMPs, weakly interacting massive particles. Wukong, known formally as the Dark Matter Particle Explorer (DAMPE), is designed to detect the signal that comes from WIMPs. WIMPs are lazy, slow-moving particles that collide rarely, but when they do, they could form pairs of electrons and positrons—the antimatter counterparts of electrons—some of the particles that make up cosmic rays. An uptick in the number of these pairs would be detected as a distinct bump in a survey of cosmic rays—like the one Wukong identified.

The DAMPE findings, which were released last month in Nature, line up nicely with other space-based experiments on dark matter in the last decade, an exciting prospect in a field that depends on measuring the same thing more than once. But “that is not to say that this is an indication of dark matter,” said Douglas Finkbeiner, an astrophysicist at Harvard who studies dark matter. There are many possible interpretations. The signal detected by DAMPE and others come from all directions in the sky, so scientists can’t pinpoint a source. The unusual cosmic rays could come from supernovae, the powerful deaths of stars, or pulsars, bright, fast-rotating stars.

For now, the DAMPE result, while tantalizing, is inconclusive. For scientists, it provides another data point in the search for dark matter. But for China in particular, the findings mean something more. DAMPE is the country’s first mission dedicated exclusively to astronomy and astrophysics, and within two years it has returned a promising result. The successful showing makes China a fierce participant—or, depending on whom you ask, competitor—in the field.

“The way I see it, we are making strides toward solving this big cosmic puzzle,” said Priyamvada Natarajan, a theoretical astrophysicist at Yale. “I don’t feel restricted by international borders in that sense. But it doesn’t go unnoticed that it is a Chinese satellite that did it.”

When people talk about China’s ambitions in space, the discussion usually focuses on activities that have some military implications—like the launch of spy satellites—rather than scientific ones. Headlines shout about a “space race” between China and the United States, and some American defense officials rattle their sabers alongside calls for increased capabilities in low-Earth orbit. The thought of Chinese spacecraft near U.S. assets in orbit is, from a national-security perspective, far more unnerving than a science mission for a particle that may not even exist.

“Unless you’re in a really intense Star Trek fantasy, it’s not going to lead you into a weapons capability,” said Joan Johnson-Freese, a national-security affairs professor at the U.S. Naval War College who has studied space security for 20 years.

China has invested heavily in space science in recent years. The country started building the world’s most powerful radio telescope in 2011, edging the famed Arecibo Observatory in Puerto Rico out of the top spot. In the fall of 2016, the telescope, the 500-Meter Aperture Spherical Radio Telescope (FAST) started making observations. It discovered two new pulsars in its first year of operations, and stands to be the leading instrument in the search for intelligent extraterrestrial life for years to come. In 2013, China landed a rover on the moon to poke around the surface. In June of this year, the Hard X-Ray Modulation Telescope (HXMT), a space observatory to study black holes and neutron stars, joined DAMPE in orbit.

The choice to invest in these particular fields have been very deliberate, according to Johnson-Freese. “China likes to be in the record books like everyone else,” she said, but the country can’t compete in areas of space exploration where the United States and other countries have long dominated. Instead, the Chinese have gone after realms in which no country has yet made a definitive triumph—like the search for dark matter.

In March 2016, a few months after DAMPE launched, Chang Jin, the mission’s chief scientist, said the search for the mysterious substance “tops the basic frontier projects of science listed by the United States, Europe, China, and Japan.”

“Any progress in dark-matter research will probably bring a breakthrough in physics,” Chang said.

While a breakthrough by the Chinese—a breakthrough by any group of scientists in any nation, really—would be cause of celebration in the astrophysical community, the merriment would feel thorny for some.“If China were to get a Nobel Prize in science, would that mean that the United States suddenly lost all of its lead? No,” Johnson-Freese said. “But I can see that there would be a lot of scientists who would say, well, this is going to become a Chinese matter of expertise. We’re going to depend on their science for us to do work.”

The isolation from a potential breakthrough likely will be felt most by American scientists, thanks to a law passed in 2011 that prohibits NASA from working with China’s space agency. There’s some irony there, given that one of the earlier experiments that noticed the same, strange blip in a survey of cosmic rays—the signal that scientists hope betrays the existence of dark matter—came from a collaboration between China, the United States, and other countries, the Alpha Magnetic Spectrometer, mounted on the International Space Station just one month after Congress approved the ban.

China has found opportunities for collaboration elsewhere. Scientists from institutions in Geneva and Italy are working on the DAMPE mission, and Chinese officials are in talks with the European Space Agency about building an outpost on the moon together. “These congressional restrictions presume that forbidding contact will slow the pace of Chinese progress,” Gregory Kulacki, a senior analyst and China project manager at the Union of Concerned Scientists, an American nonprofit group, said in an email. “Projects like FAST and DAMPE prove beyond a shadow of a doubt that presumption is mistaken.

Eventually, even China’s ambitions in particle physics will be subject to the scrutiny usually reserved for its more secretive, space-based military operations.“It seems no matter what China does in space, even if it is to make a significant scientific contribution to our understanding of the universe, some people in the United States will perceive it as a threat,” Kulacki said. But science is not a zero-sum game, he said, and the scientists themselves “understand who is to blame.”

After decades of looking for elusive dark-matter particles, the effort feels like it is at an impasse, Natarajan said. The hunt for WIMPs, the leading candidates, has repeatedly turned up empty, and astrophysicists are trying to broaden their search methods. Scientists are hopeful about more results from DAMPE, which is expected to last five years.

“We’ve all been thinking about this for so long, it’s such an embarrassment,” Natarajan said. “That anyone is making progress is super exciting.”