We present the first statistically significant detection of cosmic structure using broadly distributed hydrogen radio emission. This is accomplished using a cross-correlation with optical galaxies. Statistical noise levels of 20 μK are achieved, unprecedented in this frequency band. The signal is consistent with arising from the collective flux of known populations of galaxies. This lends support to the idea that large volumes of the Universe can be rapidly mapped without the need to resolve individual faint galaxies, enabling precise constraints to dark energy models. We discuss strategies for improved intensity mapping.