Evidently, doom from above may come unexpectantly, as scientists are discussing the existence of “mirror matter” which cannot be seen by normal means. Some astronomers believe that objects, be they comets or whole solar systems, could be comprised of this “mirror matter”.
While little evidence of this invisible substance exists, some believe that it would explain certain collisions of falling extraterrestrial bodies that haven’t left a significant visible trace behind. Similarly, it would explain certain “missing” comets that have been tracked by astronomers that seem to disappear.
Either way, the idea is pretty damned cool, and could be attributed to all sorts of crazy supernatural phenomena.