When you're stating an opinion that goes against what someone else just said, you can use the phrase "Yeah, I don't know..."

In a lot of English-speaking cultures, it's common for people to talk about their opinions on issues and to disagree with each other. But phrases like this one are important when disagreeing to let the listener know the level of your disagreement. When you use "Yeah, I don't know..." it doesn't sound like your disagreement is angry or forceful. It sounds like you're just stating your opinion, which happens to be different from the other person's opinion.