Back in April, Fermilab physicists detected a strange bump in their data that couldn't be explained by any known particles. The mystery remains unsolved, and it's looking more and more likely that we're on the verge of a major discovery.
We covered the initial announcements here and here, which offer more extensive background on the initial discovery. Very basically, the initial discovery was made during background analysis of the production of gauge boson pairs, which are elementary force-carrying particles. (If you need a refresher on the elementary particles, check out our field guide.)