The universe's invisible majority
Direct observations of galaxies and gas account for roughly 5 percent of the universe's total mass-energy: protons, neutrons, and electrons (baryonic matter). The remaining 95 percent is invisible to telescopes. Of that, 27 percent is dark matter, detected only through gravitational effects on galaxy rotation and light bending. The remaining 68 percent is dark energy, which drives the accelerating expansion of space itself.
This composition was determined by combining observations of galaxy clustering, supernova distances, and the cosmic microwave background temperature fluctuations. No single measurement suffices; the picture emerged from convergence across three independent techniques by the early 2000s.
What we know and don't know about dark components
Dark matter's nature remains unresolved after five decades of searching. Leading candidates include weakly interacting massive particles (WIMPs), which would interact only through gravity, and axions, hypothetical particles proposed to solve a different problem in particle physics. Earth-bound detectors deep underground seek direct collisions with dark matter particles passing through Earth; so far, no definitive detection exists.
Dark energy is even more mysterious. Its properties are consistent with Einstein's cosmological constant, a uniform energy density filling all space. But why is dark energy's strength so weak compared to quantum field theory predictions? This mismatch, the cosmological constant problem, remains one of physics' largest unsolved mysteries.