  The 2020 announcement by LHCb of a narrow structure $X(6900)$ in the
di-$J/\psi$ spectrum -- a potential $c\bar c c\bar c$ state -- has opened a new
era in hadronic spectroscopy. In this talk, we briefly survey theory works
preceding this event, examine key features of the observed spectrum, and then
discuss how subsequent theory studies (including with the author's own
dynamical diquark model) have interpreted these features. We conclude with
proposals for experiments capable of distinguishing competing interpretations.
