Difference between "World" and "Universe"

I am with the people.

To me as well, World = Earth while the Universe = earth and space including the great unknown.

The Oxford seems to agree.

Origin of World: Old English w(e)oruld, from a Germanic compound meaning 'age of man'; related to Dutch wereld and German Welt.

Origin of Universe: late Middle English: from Old French univers or Latin universum, neuter of universus 'combined into one, whole', from uni- 'one' + versus 'turned' (past participle of vertere).

I'd say the difference depends on context. The most literal senses of world versus universe are, respectively, the planet Earth versus all that exists. However, for practical purposes, it is often clear that all instances of a thing that could possibly exist must exist on our planet, so that the difference becomes irrelevant.

You are the most beautiful man in the world.

Since there are hardly any men outside Earth, the words world and universe could be used interchangeably here.

This must be the highest mountain in the universe.

Since there are mountains on Mars, you could not substitute world here.

In many expressions where it doesn't matter, the word world is used because it is the more common word.