  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 $\alpha_{d} =
100 \pm 10$ degrees and declination $\delta_{d} = -24_{-13}^{+12}$ degrees.
That direction indicates an extragalactic origin for these ultra-high energy
particles.
