Cosmic rays are atomic nuclei arriving from outer space that reach the highest energies observed in nature .
Clues to their origin come from studying the distribution of their arrival directions .
Using 3 { \times } 10 ^ { 4 } cosmic rays above 8 { \times } 10 ^ { 18 } electron volts , recorded with the Pierre Auger Observatory from a total exposure of 76,800 square kilometers steradian year , we report an anisotropy in the arrival directions .
The anisotropy , detected at more than the 5.2 \sigma level of significance , can be described by a dipole with an amplitude of 6.5 _ { -0.9 } ^ { +1.3 } % towards right ascension \alpha _ { \text { d } } = 100 \pm 10 degrees and declination \delta _ { \text { d } } = { -24 _ { -13 } ^ { +12 } } degrees .
That direction indicates an extragalactic origin for these ultra-high energy particles .