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Visible matter in the universe consists of 12 fundamental particles, with neutrinos being the lightest and most abundant yet least visible. These "little neutral ones" pass through Earth constantly, leaving no trace due to their low energy. Physicists study them because they can penetrate almost anything, making them ideal for understanding how particles interact and why the universe is made mostly of matter rather than equal parts matter and antimatter. Detecting them requires specialized materials at industrial scales, a challenge addressed by the IceCube Neutrino Observatory located at the South Pole.

Scientists first proposed neutrinos in 1930 but did not discover them until using liquid near a nuclear reactor core. The 2026 Nobel Prize in physics was awarded to Francis Halzen for work on IceCube, which detects neutrinos from deep space that travel through Earth. Previous discoveries included neutrinos created by particle accelerators, cosmic rays, and the Sun. Neutrinos come in three types: electron, muon, and tau. They were found to oscillate, or change types, proving they have a tiny nonzero mass.

Experiments like Super-Kamiokande in Japan and Sudbury Neutrino Observatory in Canada solved the mystery of why fewer solar neutrinos were detected than expected. Super-Kamiokande used 110 million pounds of water to observe atmospheric neutrinos changing type, while Sudbury used heavy water to detect all types from the Sun. Together, they proved neutrinos oscillate as they travel through space.

Francis Halzen and colleagues realized that powerful astrophysical events like supernovas should produce very energetic neutrinos. To catch these rare particles, a detector needed a volume of about one cubic kilometer. The team used clear deep ice at the South Pole, building IceCube with 5,000 optical sensors over seven years. In 2013, IceCube detected high-energy astrophysical neutrinos, and later confirmed their origin by matching them with gamma rays from space telescopes.