The mystery event was a "firefly" type flare some 2 billion light years away from Earth. The event occurred in the middle of a void. Nothing should have been there, and yet, there this flare was, lasting 100 days or so before fading away into nothingness. That's about 80 days longer than a traditional super nova.
Today, however, astronomers might have an answer: It was "just" your average run-of-the-mill rogue "free floating" black hole eating a star that was, until said black hole devoured it, residing in a galaxy to dim to view using existing technology.
Lucky for us, one of Hubble's new additions could help us find more "fireflies" in the void. The Wide Field Camera 3, installed by members of the Atlantis crew earlier this month, might be able to determine if there was actually a host galaxy around the mysterious flare that was just too faint to see (thereby making this one-of-a-kind flare a bit more common). [New Scientist]