It's because the universe is expending that we have a little something we like to call "time", or in other words: the 4th dimension.
As for the nothing/something part, I'm guessing we still aren't advanced enough to wrap around mind over it. The fact that we aren't able to understand something doesn't mean it doesn't exist. For exemple: we still don't know yet much about the "pre-bigbang era", if there ever was one, but that doesn't mean we should explain it with some fairy tale story. We just need to be patient and understand our universe step by step.
But one thing you can wrap your mind around, is that space isn't empty, even the blackness you see. To put it simply, there is "black matter" everywhere in between space entities (planet, stars etc), this black matter conveys forces like gravity. Now, for what's outside our universe, we don't know. Might aswell be nothing, which there is nothing wrong with it. But hopefully, we'll get to understand that during my lifetime. If not, too bad for me.