  The benchmark hot Jupiter HD 189733b has been a key target to lay out the
foundations of comparative planetology for giant exoplanets. As such, HD
189733b has been extensively studied across the electromagnetic spectrum. Here,
we report the observation and analysis of three transit light curves of HD
189733b obtained with {\Hubble}/STIS in the near ultraviolet, the last
remaining unexplored spectral window to be probed with present-day
instrumentation for this planet. The NUV is a unique window for atmospheric
mass-loss studies owing to the strong resonance lines and large photospheric
flux. Overall, from a low-resolution analysis ($R=50$) we found that the
planet's near-ultraviolet spectrum is well characterized by a relatively flat
baseline, consistent with the optical-infrared transmission, plus two regions
at $\sim$2350 and $\sim$2600 {\AA} that exhibit a broad and significant excess
absorption above the continuum. From an analysis at a higher resolution
($R=4700$), we found that the transit depths at the core of the magnesium
resonance lines are consistent with the surrounding continuum. We discarded the
presence of \ion{Mg}{ii} absorption in the upper atmosphere at a
$\sim$2--4$\sigma$ confidence level, whereas we could place no significant
constraint for \ion{Mg}{i} absorption. These broad absorption features coincide
with the expected location of \ion{Fe}{ii} bands; however, solar-abundance
hydrodynamic models of the upper atmosphere are not able to reproduce the
amplitude of these features with iron absorption. Such scenario would require a
combination of little to no iron condensation in the lower-atmosphere,
super-solar metallicities, and a mechanism to enhance the absorption features
(such as zonal wind broadening). The true nature of this feature remains to be
confirmed.
