How did the theory of social Darwinism justify European imperialism during the 19th century?

Social Darwinism was pretty much survival of the fittest. And Europe was a major world power in the 19th century. Europe was the 'strong' and the rest of the world was the 'weak' and that is why they 'survived.' I'm sorry of that doesn't make any sense, I tried to explain it as briefly as possible.