In the last 50 years of astrochemical research, the realm of one-dimensional carbon chemistry (i.e. carbon chain molecules) has been well explored. Life, however, relies on two and three-dimensional carbon - branches, rings, bridges, and so forth. Here, we present the first rotational detection of a six-membered ring molecule in the interstellar medium (ISM), benzonitrile (c-C$_6$H$_5$CN), using deep Green Bank Telescope observations of TMC-1 combined with high-precision laboratory spectroscopy. We then explore the formation chemistry of this two-dimensional carbon molecule using a combined laboratory, quantum chemical, and modeling approach. We demonstrate the synthesis of cyclic species (benzene [c-C$_6$H$_6$] and benzonitrile) from simple, acyclic precursors, providing definitive evidence for facile bottom-up generation of two-dimensional carbon chemistry in the ISM. The results show that benzonitrile can already be used as a reliable proxy for the presence of benzene in the ISM, and that there may exist a much larger array of 'hidden' aromatic species just beyond the current sensitivity of spectral surveys.