What Were The 'FOO Fighters' During The Second World War?

During the Second World War (1939-1945), the name 'FOO Fighters' was given to the bogies that were often seen by Second World War pilots or flight crews. These objects were spherical, circular, disc-like or wedge-like in shape, and sometimes appeared to glow, shine or reflect a high degree of bright illumination.

Though these FOO Fighters never attacked and did not come into any sort of contact or attempt to make any sort of interaction with the flight crews, they were often sighted and reported by crews belonging to both the groups of countries that were at war with each other at the time, the Allied Forces and the Axis Powers. They were extremely fast and easily manouvrable. They were unpredictable at times and were often also known to display feats that were extremely unconventional, such as instantaneous and rapid acceleration and deceleration, ascending and descending at a very high speed, etc.