This expression gets me every time! Gäller is such a difficult word in Swedish...
'What is the matter?' and 'What is the matter about?' mean two very different things. The first means 'Is there something wrong?' and the latter 'What is this about?' Yet they are both offered as a translation. Does Vad gäller saken? really mean both things?
No, "Vad gäller saken?" only means "What is this about?"/"What is the matter about?".
So, if "Vad gäller saken?" means "what is [the thing] about?", then how would you ask "hey, what's wrong? / what's the matter?" ?
I would suggest. "What's wrong?" = "Vad är problemet?", but it's very context dependent.
Also, "what's the matter?" = "Hur är det med dig?". (I.e. "how are you?")
In Hamlet Act II, Scene II, 191-194, Shakespeare let Hamlet make use of the ambiguity in the question What is the matter?" :-)
I am still confused about how I should say "What's the matter" in Swedish. Would that be "Vad som sker?"
Doesn't work without an är; also sker is old-fashioned or formal in Swedish so you don't hear it a lot in the spoken language. Usually we just say Vad är det? lit. 'What is it?' but there are other options too like Vad är det för fel? 'What's wrong?' or even Hur är det? 'How is it?'