Look up at the night sky and you see stars, planets, and the diffuse glow of distant galaxies. All of it — every atom of hydrogen, every photon of light, every planet and moon and black hole — constitutes just 5% of the universe's total energy content. The other 95% is something else entirely: a dark, invisible scaffolding that shapes the cosmos yet refuses to announce itself. We call it dark matter and dark energy, and decades of searching have not yet told us what either of them truly is.

What Is Dark Matter?

Dark matter is matter that does not emit, absorb, or reflect light. It passes through ordinary matter almost without interaction, which is why detectors buried deep underground — shielded from cosmic rays and background radiation — have so far come up empty-handed. Despite its invisibility, dark matter reveals itself through gravity: it bends light, pulls galaxies together, and gives clusters of galaxies far more gravitational mass than their visible stars and gas could ever provide.

The most widely accepted candidate remains the WIMP — a Weakly Interacting Massive Particle. WIMPs would be perhaps 10 to 1,000 times the mass of a proton, interact only via gravity and the weak nuclear force, and be produced in the right abundance during the Big Bang to explain what we observe today. Experiments like LUX-ZEPLIN (LZ), a 10-tonne liquid xenon detector operating 1,478 metres underground in South Dakota's Sanford Underground Research Facility, have set the most stringent limits yet on WIMP interactions — but no confirmed signal has emerged.

The Evidence from Galaxy Rotation Curves

The first strong evidence for dark matter came from astronomer Vera Rubin and her collaborator W. Kent Ford in the 1970s. When they measured the rotational velocities of stars at different distances from the centres of spiral galaxies, they found something deeply strange. According to Newtonian gravity, stars far from a galaxy's bright centre — where most of the visible mass resides — should orbit more slowly, just as the outer planets of the Solar System move more slowly than those close to the Sun. Instead, the rotation curves were flat: stars at the outskirts moved at roughly the same speed as stars near the core. The only explanation was that a vast, invisible halo of matter extended far beyond the visible disc, providing extra gravitational pull at every radius.

The evidence for dark matter has since grown overwhelming and comes from multiple, independent lines of observation:

  • Gravitational lensing: Clusters like the Bullet Cluster show visible gas displaced from the gravitational mass, with the dark matter halo passing through a collision while hot gas was slowed by electromagnetic interactions.
  • The cosmic microwave background: The precise pattern of temperature fluctuations measured by the Planck satellite is best explained by a universe that is roughly 26.8% dark matter by energy density.
  • Large-scale structure: Computer simulations of cosmic structure formation — using programs like the Illustris-TNG suite — only reproduce the observed web of filaments, voids, and galaxy clusters when dark matter is included.
  • Galaxy cluster mass: The velocities of galaxies orbiting within clusters, measured since Fritz Zwicky's work in 1933, imply total masses far exceeding visible matter.

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Dark Energy and the Accelerating Universe

If dark matter was the first great shock of modern cosmology, dark energy was the second — and in some ways the more disturbing one. In 1998, two independent teams — the High-Z Supernova Search Team and the Supernova Cosmology Project — were using Type Ia supernovae as standard candles to measure how the universe's expansion had changed over cosmic history. The expectation was clear: gravity should be slowing the expansion, so distant supernovae should appear brighter than a naive extrapolation would predict. Instead, they appeared dimmer. The universe was not decelerating. It was accelerating.

The universe is not merely expanding — it is accelerating away from itself, driven by a pressure no telescope has ever seen and no equation fully explains.

This discovery earned Saul Perlmutter, Brian Schmidt, and Adam Riess the 2011 Nobel Prize in Physics. The acceleration is attributed to dark energy, which makes up approximately 68.3% of the universe's total energy content. In Einstein's equations of general relativity, dark energy can be represented as a cosmological constant — a term Einstein himself introduced and then abandoned, calling it his greatest blunder. It turns out the universe had the last laugh: the cosmological constant, now associated with the energy density of empty space, appears to be real after all.

What dark energy actually is remains one of the deepest open problems in physics. Its measured value is roughly 120 orders of magnitude smaller than quantum field theory predicts for vacuum energy — a discrepancy so catastrophic it is sometimes called the worst prediction in all of science. Alternative models propose that dark energy is not a constant but a dynamic field called quintessence, one that evolves over time and could change the ultimate fate of the universe.

The Search for Answers

The scientific response to this double mystery has been extraordinary in its ambition. The Euclid space telescope, launched by the European Space Agency in July 2023, is mapping the shapes and distributions of billions of galaxies out to 10 billion light-years, probing both dark matter clustering and the effect of dark energy on large-scale structure. The Vera C. Rubin Observatory in Chile — named in honour of the astronomer whose rotation-curve work helped establish the dark matter paradigm — will image the entire southern sky every few nights for ten years, building the deepest and widest survey of the dynamic universe ever attempted.

Underground, the next generation of WIMP detectors is pushing sensitivity to even fainter interactions. If WIMPs do not appear before detectors begin to pick up the irreducible background from solar neutrinos — the so-called neutrino floor — the field may be forced to abandon WIMPs entirely and pivot to alternatives like axions, sterile neutrinos, or primordial black holes.

For now, 95% of the universe remains a mystery written in invisible ink. We know its mass, we know its distribution, we know the extraordinary precision with which it shapes everything we can see — and we do not know what it is. Few problems in all of science carry a higher price on their solution.