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A novel ultrafast terahertz probe has traced dark composite particles and confirmed, for the first time, that their formation occurs on the femtosecond timescale in single-walled carbon nanotubes. Dark particles, known as dark excitons, are composite pairs of quasielectrons and electron holes. Single-walled carbon nanotubes have unique electrical properties governed by the presence of dark and bright particles. Because dark excitons cannot emit a photon like their bright counterparts, they do not interact with visible light, rendering them difficult to trace.