Though its natural to try and think of the universe expanding into something, like a balloon expanding is expanding within the room you're in (assuming you're the one blowing it up) it's not necessarily true of space-time. It's quite an abstract concept and one which is hard to get your head around if you're unfamiliar with various notions within non-Euclidean geometry and relativity.
Unfortunately, and I don't mean this in an insulting way, if you were provided with the details you'd not understand them. Too often (and I'm not saying you're doing this, I'm just commenting on a common thing) certain people online will work by the principle of saying "That explanation is too vague, so I don't accept it. And that detailed explanation is over my head, so I don't accept it". It's catch 22, their own ignorance means they don't accept when they are wrong (and thus ignorant on some level). If you want to know where to start reading to eventually understand the details then I can suggest a few books but it's a long (ie years) road.