Scientific Overview
If the greatest revolutions in 20th-century physics were quantum mechanics and relativity, the greatest puzzles of 21st-century cosmology are undoubtedly dark matter and dark energy. According to the most precise cosmological observations, what we normally call "matter" — atoms, molecules, stars, planets, galaxies — constitutes only about 5% of the universe's total mass-energy. The remaining approximately 27% is dark matter, and approximately 68% is dark energy. In other words, we know virtually nothing about the vast majority of the universe.
Dark Matter
The story of dark matter begins in 1933. Swiss astronomer Fritz Zwicky, while observing the Coma galaxy cluster, discovered a puzzling phenomenon: galaxies within the cluster were moving far faster than the gravitational pull of visible matter could constrain. According to his calculations, the cluster must contain vast amounts of invisible "dark matter" to explain this high-speed motion.
Zwicky's discovery was largely ignored for decades. Not until the 1970s, when American astronomers Vera Rubin and Kent Ford studied the rotation curves of spiral galaxies, did the evidence for dark matter become undeniable. According to Newton's law of gravity and Kepler's laws, orbital velocities of stars in a galaxy's outskirts should decrease with distance — just as outer planets in our solar system orbit more slowly than inner ones. But Rubin and Ford found that stars in the outer reaches of spiral galaxies orbit at nearly the same speed as inner stars. This meant galaxies contain vast amounts of invisible mass distributed far beyond the visible stars and gas.
Since then, evidence for dark matter has continued to accumulate. Gravitational lensing — massive objects bending light from background sources — reveals gravitational effects far exceeding what visible matter alone could produce. The 2006 observation of the "Bullet Cluster" provided the most direct evidence for dark matter: when two galaxy clusters collided, the visible hot gas (observed via X-rays) was decelerated, while dark matter (observed via gravitational lensing) passed through nearly unimpeded. This demonstrates that dark matter barely interacts with anything besides gravity.
The cosmic microwave background radiation (CMB) — the afterglow of the Big Bang — also provides strong evidence for dark matter. Tiny temperature fluctuations in the CMB reflect minute density variations in the early universe, and the pattern of these fluctuations matches observations only when dark matter is included. The European Space Agency's Planck satellite provided the most precise measurements of the CMB, yielding the universe's composition: 4.9% ordinary matter, 26.8% dark matter, 68.3% dark energy.
What exactly is dark matter? This remains one of the great mysteries spanning particle physics and astrophysics. Leading candidates include: Weakly Interacting Massive Particles (WIMPs) — hypothetical particles interacting with ordinary matter through the weak nuclear force and gravity; axions — extremely light hypothetical particles originally proposed to solve the CP problem in quantum chromodynamics; sterile neutrinos — hypothetical neutrino variants heavier than the three known types; and primordial black holes — microscopic black holes formed in the early Big Bang.
Multiple experiments worldwide are searching for dark matter particles. Underground direct detection experiments (such as PandaX at China's Jinping Underground Laboratory and XENON in Italy) attempt to capture faint signals from dark matter particles colliding with ordinary atomic nuclei. Particle accelerators (like the Large Hadron Collider) attempt to create dark matter particles in high-energy collisions. Space telescopes and gamma-ray detectors search for radiation that might result from dark matter annihilation. To date, none of these efforts has produced a definitive detection.
Dark Energy
If the dark matter story is full of twists but gradually clarifying, the discovery of dark energy was a complete and utter surprise.
In 1998, two independent astronomical research teams — the Supernova Cosmology Project (SCP) and the High-z Supernova Search Team (HZT) — made a discovery that shocked the entire physics community while using Type Ia supernovae as "standard candles" to measure the universe's expansion rate: the expansion of the universe is not decelerating but accelerating.
This was entirely unexpected. Since Hubble's 1929 discovery of cosmic expansion, physicists had assumed gravity would gradually slow the expansion — the question was only by how much, and whether the universe would eventually stop expanding and begin contracting. No one expected acceleration.
To drive this accelerating expansion, the universe must contain some form of energy with negative pressure — an "anti-gravity" effect. This mysterious energy was named "dark energy." The simplest explanation is Einstein's cosmological constant Λ — introduced in 1917 and later retracted — an inherent energy density of the vacuum itself. Quantum mechanics predicts that vacuum is not empty but filled with "quantum foam" of continuously creating and annihilating virtual particle pairs, and this quantum vacuum energy might be the source of dark energy.
However, quantum mechanics' calculated value for vacuum energy density differs from the observed dark energy density by a factor of approximately 10¹²⁰. This is called the "cosmological constant problem," one of the most severe numerical discrepancies in theoretical physics.
Another possibility for dark energy is "quintessence" — a dynamic, time-varying scalar field. Unlike the cosmological constant, quintessence energy density can change as the universe evolves. Current observational data cannot yet distinguish between the cosmological constant and quintessence.
Applications in Three-Body
While dark matter and dark energy are not the primary scientific background of Three-Body, they play crucial roles in the grand cosmological narrative of the third novel, Death's End.
Pocket Universes and Mass Theft: One of Three-Body's most stunning cosmological settings is the concept of "pocket universes." Advanced civilizations can create miniature universes independent of the main universe, serving as refuges or living spaces. Cheng Xin and Guan Yifan enter one such pocket universe — "Universe 647" — at the story's end.
However, creating pocket universes has a fundamental problem: the matter and energy within them come from the main universe. Each pocket universe created means a loss of mass from the main universe. When countless civilizations across the universe create pocket universes for self-preservation, the main universe's total mass continuously decreases.
This setting provides an imaginative science fiction explanation for the real-world "dark energy mystery" and "cosmic mass deficit." In real cosmology, the universe's accelerating expansion suggests some "dark energy" is at work; in the Three-Body universe, the continuous loss of mass may be among the causes of the universe's anomalous behavior.
The Returners and Universal Fate: The "Returners" appearing at the end of the third novel are a super-civilization (or coalition) that broadcasts a message to the entire universe, calling on all civilizations to return mass stolen from the main universe. The Returners' logic is clear: if the main universe's mass falls below the critical threshold, the universe will expand forever, ultimately reaching heat death — a state of ultimate death where temperature approaches absolute zero, all stars extinguish, all matter decays, and all energy disperses uniformly.
Only when the universe's total mass exceeds the critical mass can gravity ultimately overcome expansion, allowing the universe to undergo the Big Crunch — collapsing into a singularity and beginning anew with a new Big Bang, a new universe. The Returners hope for precisely this cyclical rebirth.
This plot directly corresponds to real cosmological discussions about the universe's ultimate fate. In the standard cosmological model, the universe's fate depends on the ratio of its total density to critical density (the density parameter Ω). If Ω > 1, the universe eventually contracts; if Ω < 1, the universe expands forever; if Ω = 1, the universe expands infinitely at a rate approaching zero. Current observations indicate Ω is very close to 1, but dark energy's presence makes the universe's fate more complex — accelerating expansion means even if Ω > 1, the universe may still expand forever.
Dimensional Degradation and Energy Landscape: In the Three-Body cosmology, dimensional reduction is closely tied to the universe's overall energy landscape. When the universe drops from higher to lower dimensions, energy previously stored in higher dimensions is released or transformed in some form. Liu Cixin implies that during the universe's descent from ten dimensions to three, vast amounts of energy and information were permanently lost.
This setting can be seen as a science fiction response to the "dark energy problem": the universe's seemingly excess energy (dark energy) and seemingly missing matter (dark matter) may both relate to the history of dimensional degradation. In a higher-dimensional universe, this energy and matter may exist in forms that we cannot directly observe from our three-dimensional world.
The Moral Dimension of the Universe: The dark matter/dark energy narrative in Three-Body is not merely physical but ethical. The Returners' call raises a profound moral question: when you possess the ability to create pocket universes, do you have the right to steal mass from the main universe for your own survival, even if it might cause the entire universe's death?
The choice Cheng Xin faces at the story's end epitomizes this moral dilemma. She could remain in the pocket universe indefinitely, but this would mean permanently stealing five kilograms of mass from the main universe (the mass of the ecological sphere within the pocket universe). Ultimately, Cheng Xin chooses to return the ecological sphere's matter to the main universe, keeping only a small fishbowl — a decision symbolizing individual responsibility toward the universe's collective fate.
Real-World Scientific Basis
Dark matter and dark energy are among the most active research areas in contemporary physics and astronomy.
In dark matter detection, dozens of experiments are underway worldwide. China's Jinping Underground Laboratory (CJPL) is one of the world's deepest underground laboratories, where the PandaX experiment uses liquid xenon as a detection medium to search for faint scintillation and ionization signals from dark matter particles colliding with xenon nuclei. To date, all direct detection experiments including PandaX have found no confirmed dark matter signal, but they continue tightening constraints on dark matter particle properties.
CERN's Large Hadron Collider (LHC) is also searching for dark matter. The LHC's high-energy proton-proton collisions could potentially produce dark matter particles, manifesting as "missing energy" in collision products. While the LHC has not found dark matter, it has ruled out many dark matter candidate models.
In dark energy research, the Dark Energy Survey (DES) project used the Blanco telescope in Chile to perform deep imaging of approximately 5,000 square degrees in the southern sky, constraining dark energy properties through multiple methods including weak gravitational lensing, baryon acoustic oscillations, and supernovae. The European Space Agency's Euclid satellite, launched in 2023, is specifically designed to study dark energy and dark matter, performing precise measurements of billions of galaxies.
China's space station program also lists dark matter and dark energy research as important objectives. The China Space Station Telescope (CSST) plans to conduct large-scale deep surveys, with its wide field of view and high resolution providing important data for dark energy and dark matter research.
On the theoretical front, physicists are exploring various beyond-Standard-Model theories to explain dark matter and dark energy. Modified gravity theories (such as MOND — Modified Newtonian Dynamics) attempt to explain galaxy rotation curves without invoking dark matter, but face difficulties explaining cosmological-scale observations. String theory and supersymmetry predict various dark matter candidate particles, but these predictions remain experimentally unconfirmed.
The mysteries of dark matter and dark energy may require a fundamental breakthrough in physics to resolve — perhaps a "Theory of Everything" unifying gravity and quantum mechanics, or perhaps a fundamental revolution in our understanding of spacetime's nature. As Three-Body implies, the universe we observe may be merely a dimensional slice of a far richer and more complex cosmos than we can imagine.