Cosmic rays are very high energy atomic and subatomic particles that hit the earth from space. There are several which pass through you every second, but most of these are fairly low energy and come from the sun. Others come from outside the solar system and the more energy they have the rarer they are. Until about 10 years ago astronomers thought that there was a limit on the speed of these particles because they would get slowed down by hitting photons from the cosmic microwave background radiation.
Then astronomers detected what are sometimes known as "Oh-my-god!" particles in a couple of small cosmic ray experiments. One of these sub-atomic particles has a similar energy to a tennis ball travelling at 60mph despite being about 10 000 000 000 000 000 000 000 times lighter. This is about 100 million times more energy than the particles in our biggest particle accelerators.
Scientists had no idea what was creating these particles until now, to find out they built the Pierre Auger Observatory in Chilie, this is an array of detectors the size of Paris built high up in the Andies mountains. It consists of hundreds of tanks of very pure water which glows when a cosmic ray goes through it. They also look for similar ultra violet glows in the atmosphere above the detectors. A very high energy particle will hit the air and create a whole cascade of other particles which the detectors can see.