Why does buoyant force exist?

This might help. 
Imagine you are at a concert. Everyone is attracted to the band on stage. But some people are especially attracted and so they force their way forward. But everyone cannot be up close, so the less forceful people end up further back. What is the force that pushes the other people backwards? 

Buoyancy is when one substance, say air, is attracted to something more than other substances, say helium, and the force of the attraction of the first (air) creates an opposing force on the second (helium).