This theory says that any flat map of interconnecting lines may be coloured using only 4 distinct colours, such that no two ajoining areas have the same colour.

As far as I know, this theory has not been proven. Not being mathematically minded, I’m just wondering why it isn’t possible to prove this? I mean, no-one has ever created a map which can’t be coloured using just 4 colours, have they? I would have thought, especially with the aid of fast computers, that this theory would have been proven by now.