Construction of the Pierre Auger Observatory is underway in Argentina, and scientists expect to begin observing there next year. A second observatory is to be built in a few years in Utah. Together, the two facilities will cover the entire sky, looking for those ever illusive trails of debris from a cosmic collision. It's a mystery, and when you solve it, you are going to learn something new. That, he says, makes it a perfect scientific experiment.
Theories abound, of course.
Some have speculated that they are produced by black holes gobbling up their neighborhood. Others have suggested they are artifacts of the earliest universe. They apparently do not come from within our own galaxy, because they arrive uniformly from all directions. The Milky Way is not uniform, so if they originated within the galaxy more would come from some areas than others. So they must come from outside our galaxy.