The Dark Rulers of the Universe: Dark Matter and Dark Energy
Dark matter, making up 26.8% of the universe, is now being hunted in an underground lab beneath a mountain in Gangwon Province.
Dark energy, which accounts for 68.3% of the cosmos and is driving its expansion, is being sought with the help of massive telescopes.
700 Meters Underground: The Hunt for an Invisible Entity
Beneath Mount Jeombong in Yangyang, Gangwon Province, 700 meters underground, stands a small two-story laboratory belonging to the Institute for Basic Science’s (IBS) Underground Laboratory Research Group. Inside sits a two-meter-tall sodium iodide (NaI) detector—its crystal core, about the size of two bricks, is encased in layers of shielding: 3 cm of plastic, 20 cm of lead, 3 cm of copper, and a barrier of mineral oil.
Here, scientists wait for signs of WIMPs—Weakly Interacting Massive Particles—the leading candidates for dark matter.
Dark matter constitutes over a quarter of the universe and is astonishingly abundant: every second, hundreds of thousands of these particles pass through an area the size of a fingernail. “They’re streaming through our bodies and the Earth right now, unseen,” says Deputy Director Lee Hyun-su. “They remain invisible because they don’t interact with other particles.” Light, for example, is visible because photons interact with matter; particles can also be detected through electrical interactions. But dark matter interacts with nothing—not even light—making it invisible and extremely difficult to detect.
Why Deep Underground?
Detecting dark matter means patiently waiting for a rare, almost accidental event. A WIMP might occasionally collide head-on with the nucleus of a sodium iodide atom inside the detector, producing a faint flash of light—perhaps once or twice a year.
The deeper the detector is buried, and the more shielding surrounds it, the fewer false signals appear from other particles. Most particles, unlike WIMPs, will interact with matter in the rock and shielding and be eliminated. What’s left may well be dark matter.
For this reason, the world’s leading WIMP detectors are located in deep mines or caves: 1,000 meters underground in Japan, over 1,400 meters in Italy and the U.S., and 2,400 meters in Sichuan, China. Korea is also building a deeper facility in Samcheok. The stakes are immense—whoever discovers dark matter will almost certainly win a Nobel Prize.
Dark matter candidates have included massive dark stars and neutrinos, but experiments have ruled out most possibilities. WIMPs remain the strongest contender. First hypothesized in the late 1970s, they are thought to be more than 100 times heavier than protons. “If WIMPs behave as predicted,” says Lee, “it would mean we already have an almost complete understanding of the universe’s birth and evolution.”
Axions: Another Dark Matter Candidate
IBS also hosts the Axion and Precision Physics Research Center, led by Greek physicist Yannis Semertzidis. Axions, first proposed in 1977 while studying the strong nuclear force that binds protons and neutrons together, are another leading dark matter candidate.
Korean physicist Kim Jihn first suggested that axions could be “invisible,” meaning they don’t interact with ordinary matter, and that they could be the particles making up dark matter. Their name comes from a popular laundry detergent—symbolizing the idea that axions could “clean up” unresolved problems in physics.
Unlike the heavy WIMP, axions are extremely light and may transform into photons in the presence of a strong magnetic field. Experiments involve placing magnets more than 100 billion times stronger than those in space inside detectors and gradually tuning them, like a radio dial, in hopes of finding the exact magnetic conditions to trigger axion–photon conversion.
“Because we don’t know the axion’s exact mass or properties, even with powerful magnets, the search could take over a decade,” says researcher Kim Young-im. And because axions are so light, they likely cannot account for all dark matter—meaning other forms must exist too.
The Biggest Mystery Since the Higgs Boson
Since the discovery of the Higgs boson in 2012, dark matter has become the next grand challenge in physics. The Higgs, theorized in the 1960s, explains how particles acquire mass, earning it the nickname “the God particle.”
CERN’s Large Hadron Collider (LHC), which discovered the Higgs, now aims to create dark matter in the lab by reproducing conditions similar to those of the early universe. The difficulty is that even if WIMPs are produced, identifying them among the myriad particles generated in high-energy collisions is extraordinarily challenging.
“No one has ever seen dark matter, but we know it must exist,” says Kim Young-im. Simulations of the early universe without dark matter produce almost no stars or galaxies—suggesting that our very existence depends on it.
The Return of Einstein’s “Biggest Blunder”: Dark Energy
One hundred years ago, in November 1915, Einstein published his theory of general relativity, showing that gravity bends space-time. Believing the universe to be static, he introduced a “cosmological constant” into his equations—a repulsive force balancing gravity.
But in 1923, Edwin Hubble discovered that galaxies are receding from Earth, proving the universe is expanding. Einstein abandoned his constant, calling it his “greatest mistake.” Yet, in 1998, NASA’s Adam Riess and colleagues found that the universe’s expansion is accelerating—a discovery that earned them the 2011 Nobel Prize and revived Einstein’s repulsive force under the name “dark energy.”
Dark energy now accounts for 68.3% of the universe’s total energy, but its true nature remains unknown.
Candidates: Vacuum Energy and Quintessence
One possibility is vacuum energy—energy inherent in “empty” space, as allowed by quantum mechanics. Another is quintessence, a hypothetical field named after Aristotle’s “fifth element,” which may have lain dormant in the early universe before later driving its accelerated expansion.
Astronomers test these ideas in two main ways:
-
Photometric surveys, which capture images of galaxies to measure how much their light is bent by gravity. Less bending could indicate the presence of repulsive dark energy counteracting gravity. Projects like the Dark Energy Survey (DES) in Chile and the upcoming Large Synoptic Survey Telescope (LSST) aim to map this effect.
-
Spectroscopic surveys, which split light into its component wavelengths to measure how fast galaxies are moving away. The Dark Energy Spectroscopic Instrument (DESI) in Arizona, starting in 2018, will map the universe in 3D to pinpoint dark energy’s influence.
Testing Einstein in Space
Einstein’s theory continues to face scrutiny. The European Space Agency’s LISA Pathfinder satellite, launched in December 2015, aims to detect gravitational waves by measuring minuscule changes in distance between two gold–platinum cubes in near-perfect free fall.
Other experiments repurpose misaligned GPS satellites to test time dilation—Einstein’s prediction that time runs slower in stronger gravitational fields.
In the end, our understanding of the universe seems to begin and end with Einstein.
