Difficulty is a synonym of Obstacle. Obstacle is a synonym of Barrier. 
Galaxy is a type of Collection. Galaxy is related to Big. Big is related to Liberty. Liberty is related to Concept. 
Difficulty <--Synonym--> Obstacle <--Synonym--> Barrier 
Collection <--IsA-- Galaxy <--RelatedTo--> Big <--RelatedTo--> Liberty <--RelatedTo--> Concept 