Learn Swedish: Journalisten undrar om ryktet stämmer. - The journalist wonders if the rumor is correct.

It can mean both! In this sentence, om introduces a subordinate clause that acts as an indirect question. The journalist is essentially asking, 'Is the rumor true?'. You can translate om perfectly here as either 'if' or 'whether'.

I usually use the verb 'att vara' (to be) for things like this. Why use stämmer instead of är rätt?

You could absolutely say är sant (is true) or är rätt (is right). However, stämmer is a very natural and common Swedish verb used to say that a piece of information is correct, accurate, or 'adds up'. You will hear it a lot with things like rumors, facts, and bills.

Why is 'the rumor' written as ryktet?

'A rumor' is an ett-word in Swedish: ett rykte. To make an ett-word that already ends in a vowel definite ('the' rumor), you just add -t to the end. That gives us ryktet.

If I wanted to say the journalist wonders if the rumor is not correct, where would inte go?

It would go right before the verb: Journalisten undrar om ryktet inte stämmer. Because om ryktet stämmer is a subordinate clause, sentence adverbs like inte must come before the verb. This is often taught to learners as the 'BIFF' rule.