In an article published today in the journal Science, the Pierre Auger Collaboration has definitively answered the question of whether cosmic particles from outside the Milky Way Galaxy. The article, titled “Observation of a large-scale anisotropy in the arrival directions of cosmicrays above 8 × 1018 eV”, notes that studying the distribution of the cosmic ray arrival directions is the first step in determining where extragalactic particles originate.