In 2021, nearly simultaneous reports of the detection of indene (c-C₉H₈) toward TMC-1 were published by Cernicharo and coworkers and by Burkhardt and coworkers. The detection by the European group was made with the Yebes 40m radiotelescope as part of the Nanocosmos project, while the detection by the US group was made with the Green Bank 100m radiotelescope as part of the GOTHAM survey. The detections have not been confirmed to date with subsequent observations.

A few months prior to the reported detection of indene, Doddipatla and coworkers published work in Science Advances indicating that indene could be formed by the reaction with CH and styrene. There has evidently been no astronomical search for styrene (also known as vinyl benzene and phenylethylene) to date; it has been suggested that styrene could be formed via the reaction of the vinyl radical (C₂H₃) with benzene, but this reaction has a sizable barrier (Tokmakov & Lin reported a calculated barrier height greater than 8 kcal/mol in 2004). The detection of 2-cyano-indene was reported in October 2022 by Sita and coworkers toward TMC-1. It could form by the gas phase reaction of CN with indene.

Indene is a bicyclic compound with an aromatic 6-membered ring fused to a non-aromatic 5-membered ring. It is the first pure hydrocarbon PAH to be detected in space.