unseen material excludes atoms, it can’t possibly be anything like stars. It must be something much more slippery to have thus far escaped detection.

Scientists often classify dark matter into two distinct categories based on its thermal properties: hot and cold. Hot dark matter involves fast-moving particles, such as neutrinos. Individually, these particles have negligible mass. They are so abundant, however, that even with small nonzero masses they’d collectively produce a significant gravitational effect.

Cold dark matter, in contrast, consists of slower-moving materials. MACHOs are often placed in this category. Non-baryonic examples include various classes of hypothetical particles, called axions and WIMPs (weakly interacting massive particles). The latter is a play on words—contrasting these diminutive constituents with “manly,” stellar-sized MACHOs. Axions, WIMPS, and neutrinos are thought to be largely unseen because they interact so rarely with ordinary matter. Neutrinos, for instance, pass straight through Earth all the time. They are extraordinarily common particles, but because they are lightweight, electrically neutral, and impervious to the strong nuclear force, they have few opportunities for contact with other matter. For the most part they are oblivious to their surroundings, like someone in a sensory deprivation chamber. Their main mode of interaction lies in the weak nuclear force—through, for instance, the process of beta decay.

Identified but not fully understood in the late 19th century, beta decay is the name given to a process wherein neutrons break down into protons, electrons, and neutrinos. (Until the time of Pauli, physicists didn’t know about neutrinos; he inferred their existence through conservation principles.) It is a common process—occurring, for example, during the transformation of radioactive isotopes.