A fast radio burst (FRB) is a high-energy astrophysical phenomenon manifested as a transient radio pulse lasting only a few milliseconds. These are bright, unresolved, broadband, millisecond flashes found in parts of the sky outside the Milky Way. The component frequencies of each burst are delayed by different amounts of time depending on the wavelength. This delay is described by a value referred to as a dispersion measure. Fast radio bursts have dispersion measures which are: much larger than expected for a source inside the Milky Way; and consistent with propagation through an ionized plasma.