The Dark Universe

Every galaxy we have ever measured spins too fast for its own visible mass. The stars on the outer edge of the Milky Way move at roughly the same velocity as the stars near the center — a flat rotation curve that Newtonian gravity cannot explain without adding something we cannot see. The Swiss astronomer Fritz Zwicky first noticed the discrepancy in the Coma Cluster in 1933 and gave the invisible mass its working name: dark matter. Ninety-two years later, we still do not know what it is.

We do, however, know roughly how much of it there is. The Planck satellite measured the cosmic microwave background — the heat left over from when the universe was 380,000 years old — to such precision that the contents of the cosmos can be inventoried directly. Ordinary matter, the protons and neutrons that make up stars, planets, and everything alive, accounts for 4.9 percent of the total. Dark matter accounts for 26.8 percent. The remaining 68.3 percent is something even stranger: dark energy, a negative pressure stretching the fabric of space, accelerating the expansion of the universe, and ensuring that in roughly a hundred billion years every galaxy outside our own local group will have receded beyond the cosmological horizon and out of sight.

The mainstream candidate for dark matter is the WIMP — a Weakly Interacting Massive Particle predicted by extensions of the Standard Model. After thirty years of underground searches, the largest WIMP detectors have produced increasingly tight null results. Then, in November 2025, the Japanese physicist Tomonori Totani published a one-author paper in Physical Review D claiming to see a faint gamma-ray excess in fifteen years of Fermi-LAT archival data — a signature exactly where a halo of WIMPs annihilating in the Milky Way should glow. The result is being independently checked. If it holds, the longest-standing problem in physics has just cracked open.

The articles below cover what dark matter and dark energy do, where they came from, the anomalies that suggest the standard model of cosmology is incomplete, and the precision-clock experiments now testing whether Einstein's geometry was the last word.