The basic principle of the 'Core-Periphery' theory is that as general prosperity grows worldwide, the majority of that growth is enjoyed by a 'core' region of wealthy countries despite being severely outnumbered in population by those in a 'periphery' that are ignored.
There are many reasons why this global structure has formed, but generally there are many barriers, physical and political, that prevent the poorer citizens of the world from participating in global relations.