Within the established framework of structure formation, galaxies start assystems of low stellar mass and gradually grow into far more massive galaxies.The existence of massive galaxies in the first billion years of the Universe,suggested by recent observations, appears to challenge this model, as suchgalaxies would require highly efficient conversion of baryons into stars. Aneven greater challenge in this epoch is the existence of massive galaxies thathave already ceased forming stars. However, robust detections of early massivequiescent galaxies have been challenging due to the coarse wavelength samplingof photometric surveys. Here we report the spectroscopic confirmation with theJames Webb Space Telescope of the quiescent galaxy RUBIES-EGS-QG-1 at redshift$z=4.896$, 1.2 billion years after the Big Bang. Deep stellar absorptionfeatures in the spectrum reveal that the galaxy's stellar mass of$10^{10.9}\,M_\odot$, corroborated by the mass implied by its gas kinematics,formed in a short $340\,$Myr burst of star formation, after which starformation activity dropped rapidly and persistently. According to currentgalaxy formation models, systems with such rapid stellar mass growth and earlyquenching are too rare to plausibly occur in the small area probedspectroscopically with JWST. Instead, the discovery of RUBIES-EGS-QG-1 impliesthat early massive quiescent galaxies can be quenched earlier or exhaust gasavailable for star formation more efficiently than currently assumed.

AGC 114905 is a dwarf gas-rich ultra-diffuse galaxy seemingly in tension withthe cold dark matter (CDM) model. Specifically, the galaxy appears to have anextremely low-density halo and a high baryon fraction, while CDM predictsdwarfs to have very dense and dominant dark haloes. The alleged tension relieson the galaxy's rotation curve decomposition, which depends heavily on itsinclination. This inclination, estimated from the gas (neutral atomic hydrogen,HI) morphology, remains somewhat uncertain. We present unmatched ultra-deepoptical imaging of AGC 114905 reaching surface brightness limits $\mu_{\rmr,lim} \approx 32$ mag/arcsec$^2$ ($3\sigma$; 10 arcsec $\times$ 10 arcsec)obtained with the 10.4-m Gran Telescopio Canarias. With the new imaging, wecharacterise the galaxy's morphology, surface brightness, colours, and stellarmass profiles in great detail. The stellar disc has a similar extent as thegas, presents spiral arms-like features, and shows a well-defined edge. Starsand gas share similar morphology, and crucially, we find an inclination of$31\pm2^\circ$, in agreement with the previous determinations. We revisit therotation curve decomposition of the galaxy, and we explore different massmodels in the context of CDM, self-interacting dark matter (SIDM), fuzzy darkmatter (FDM) or Modified Newtonian Dynamics (MOND). We find that the latterdoes not fit the circular speed of the galaxy, while CDM only does so with darkhalo parameters rarely seen in cosmological simulations. Within theuncertainties, SIDM and FDM remain feasible candidates to explain the observedkinematics of AGC 114905.