  The convective dredge-up of carbon from the interiors of hydrogen-deficient
white dwarfs has long been invoked to explain the presence of carbon absorption
features in the spectra of cool DQ stars ($T_{\rm eff} < 10{,}000\,{\rm K}$).
It has been hypothesized that this transport process is not limited to DQ white
dwarfs and also operates, albeit less efficiently, in non-DQ hydrogen-deficient
white dwarfs within the same temperature range. This non-DQ population is
predominantly composed of DC white dwarfs, which exhibit featureless optical
spectra. However, no direct observational evidence of ubiquitous carbon
pollution in DC stars has thus far been uncovered. In this Letter, we analyze
data from the Galaxy Evolution Explorer (GALEX) to reveal the photometric
signature of ultraviolet carbon lines in most DC white dwarfs in the
$8500\,{\rm K} \leq T_{\rm eff} \leq 10{,}500\,{\rm K}$ temperature range. Our
results show that the vast majority of hydrogen-deficient white dwarfs
experience carbon dredge-up at some point in their evolution.
