The longstanding mystery of their origin, which has further been compounded by the fact that only 17 such events have ever been detected, has spawned a plethora of scientific (and not so scientific) theories, including the occasional speculation that aliens are responsible for them. (ibtimes.com)
The leading suspects in the half - century old mystery of the origin of the highest - energy cosmic particles in the universe were in galaxies called «active galactic nuclei,» which have a super-radiating core region around the central supermassive black hole. (sciencedaily.com)
The longstanding mystery of their origin, which is further compounded by the fact only about two dozen such events have ever been detected, has spawned a plethora of scientific (and some that sound not so scientific) theories, including the occasional speculation that aliens are responsible for them. (ibtimes.com)