To explain this a little further, ATLAS is one of the four detectors located around the Large Hadron Collider ring at CERN. Two protons beams, travelling in opposite directions, collide inside the ATLAS detector. Before this collision, the momentum in a direction perpendicular to the proton beams is 0. Therefore, after the collision, the momentum in a direction perpendicular to the proton beams must also be 0 since momentum is always conserved in collisions. So, if ATLAS detects any particles travelling in a direction perpendicular to the original proton beams there must be other invisible particles (possibly dark matter) travelling in a different direction so that overall the momentum in a direction perpendicular to the original proton beams adds up to 0.

Just to clarify, these Population II stars are not just found in the Milky Way but all over the universe! They are often found in 'globular clusters' (spherical groups of stars found within many galaxies). The age of these stars is similar to the age of the Milky Way so Population II stars were formed around 13.6 billion years ago.