"Dark matter" is really just a name for the cause, whatever it turns out to be, of extra gravity far outweighing the gravity of the matter we can see. It isn't an explanation, it's an observation. Like the observation that galaxies rotate too fast, this observation- gravitational lensing too great for the observed matter that keeps pace with the galactic core not the dust, is an anomaly that needs explaining and has implications for the hypothesies of what could cause the effects attributed to dark matter.
From the article:
"For the first time in history, astronomers caught dark matter at work. "These results prove that dark matter exists," declared Clowe. "
So dark matter is being presented as a theoretical leap that was previously not clearly observed.
Also from wikipedia:
"Dark matter is a hypothetical type of matter distinct from baryonic matter (ordinary matter such as protons and neutrons), neutrinos and dark energy. The existence of dark matter would explain a number of otherwise puzzling astronomical observations."
Dark matter isn't an observation, it's a proposed, exotic, theoretical state of matter, that could explain observations.
When written so briefly it sounds too good to be true. I think many readers wouldn't so simply accept that dark matter is the only explanation for the mass discrepancy.