The observational confirmation of dark matter. [LINK] Far and away the most important science story for the month, and will definitely be in the top 10 for the year.
This is a great example of how science works. A set of physical rules seemed to make sense, but something didn't fit. Physicists thought they understood how gravity worked (at large scales), but galaxies and clusters of galaxies didn't seem to obey it. It was as if there were more mass that could be measured. A lot of explanations were presented, but one called dark matter seemed to come to the fore. The idea is that there is matter that interacts gravitationally, but we can't actually see it. This conjecture seems to be proved now with observations.
Guess what, science works!
Someone tell Congress.