I would've said a neutrino but that's already been suggested, technically something so far away it's impossible for the person to reach in time like galaxy HD1 would make it impossible to be killed by it since the person would need to travel an unfeasible distance and would die of old age with current tech. If your going by fictional rules like a futuristic anti aging serum, allowing teleportation of the object or to the object, or some sort of unrealistic space ship there's really no possible answer