Dark Matter, Dark Energy and What the Universe Is Made Of

Galaxies spin too fast for the gravity of their visible stars and gas. Rotation speeds stay high far out in the disk instead of falling off, which means unseen mass surrounds every spiral in a vast halo. Clusters show the same problem: member galaxies move so quickly that only five to ten times more mass than we see could hold the group together. Gravitational lensing maps the extra mass directly and agrees.

Decades of searches have not caught a single dark matter particle, yet the gravitational case keeps strengthening. Candidate particles must be slow moving, electrically neutral, and almost non interacting, otherwise they would have shown up in detectors or shaped galaxies differently. Mapping the invisible with lensing and galaxy motions is now a precision business.

Weighing the Milky Way starts with orbits. Stars near the Sun circle the center at about 220 kilometers per second, and that speed plus their distance gives the mass enclosed. The tally far exceeds all stars, gas, and dust counted inside that radius. Extending the measurement outward with satellite galaxies and distant tracers only widens the gap.

A flat rotation curve is the smoking gun. If visible matter were all there was, orbital speeds would drop with distance exactly as planets slow farther from the Sun. They do not, so a dark halo several times wider than the glowing disk must dominate our galaxy's gravity.

Three independent lines agree on the cosmic recipe. Ordinary atoms make up only about 5 percent of the total energy budget. Dark matter contributes roughly 27 percent, felt through gravity but emitting no light. Dark energy supplies the remaining 68 percent as a smooth component driving accelerated expansion.

Each share comes from different evidence: light element abundances and the microwave ripples for atoms, galaxy motions and lensing for dark matter, and distant supernovae plus cosmic geometry for dark energy.

In the late 1990s two teams measured distant exploding stars and found them fainter, and thus farther, than steady expansion allowed. The expansion is speeding up, pushed by something that acts like energy inherent in space itself. This dark energy now dominates the cosmic budget and sets our long term fate toward ever faster stretching.

Supernova evidence combined with the flat geometry from the microwave background leaves no room for a universe of matter alone. Ordinary plus dark matter sum to only about a third of the critical density, so dark energy must supply the rest. Future surveys map millions of galaxies to track exactly how the acceleration evolves.