NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) marked a watershed moment in cosmology. By mapping the cosmic microwave background, it measured the universe’s age, determined the curvature of space, and revealed that ordinary atoms make up only 4.6 % of the cosmos.
In contrast, the rest of the universe is far from empty. Dark matter accounts for 23.3 % while dark energy fills the remaining 72.1 % (NASA). Together these components comprise 95.4 % of the universe—highlighting why dark energy remains one of the biggest mysteries in modern physics.
Although WMAP launched in 2001, the clue to dark energy appeared two years earlier. In 1998, the Hubble Space Telescope observed three distant Type Ia supernovae, the farthest of which exploded 7.7 billion years ago—more than halfway back to the Big Bang (Hubblesite). Those observations showed the universe’s expansion is accelerating, contrary to the deceleration expected from gravity.
Scientists attribute this acceleration to dark energy—a force whose nature remains unknown. It must permeate the vast reaches of space to counteract gravity’s pull.
While its exact identity is unclear, several leading theories exist. One posits that dark energy is a property of space itself, in line with Einstein’s cosmological constant. This constant, often referred to as vacuum energy, would remain unchanged as the universe expands, providing a steady push against gravity.
Another hypothesis—quintessence—suggests dark energy is a dynamic field, a fluid with negative gravitational mass (NASA). Some models also explore non‑uniform distributions of dark energy or modifications to our current theory of gravity.
Unlike dark energy, dark matter is relatively better understood. Though it neither emits nor reflects light, its gravitational influence can be mapped through gravitational lensing, where the mass bends light from distant galaxies. These observations rule out ordinary matter as the culprit.
Potential candidates include supermassive black holes, massive compact halo objects (MACHOs) such as brown dwarfs, and weakly interacting massive particles (WIMPs), which would represent a primordial form of matter left over from the Big Bang.
Ongoing research seeks to pinpoint the true nature of both dark matter and dark energy, which together dominate the dynamics of our universe.
