I'd guess they were warp zones to other parts of the universe. But it's probably theories like this that got me laughed out of the scientific community. I don't think they've got very far in my absence anyway. They still haven't worked out what's beyond the universe. They try to baffle you with numbers and diagrams but they don't know really. They say 'what if the universe is everything that exists, then there can't be anything outside it.' I say 'there might be other universes'. They say 'the universe is often visualised as a three-dimensional sphere embedded in four-dimensional space'. I say 'what if you go out one end and come in the other like in Pac-Man.'
Because the universe is big. Really, really big. No, not quite that big. But big. The most distant stars we can see are only visible in the past, because the light takes so long to get to us. The light from the furthest stars started travelling when there weren't even any humans on Earth. It wasn't even called Earth then. Just a mound in space. When the light started travelling there were just little squidgy things flopping around straining to evolve.
Then one of the them strained hard enough to get legs and arms, and all the others decided they wanted some too. Others couldn't be bothered and flopped in the sea to learn how to swim. We didn't talk to them again, but after a while we produced some fingers, and thumbs, and started calling each other names. Then we started speaking in words that we made up and learned how to be sarcastic all the time. Somebody called the mound Earth (who was this?) and people drew lines and divided the place up into turf. Some of the countries don't like each other very much. Somebody invented money and convinced everyone else that they need it. We got iPods, then iPhones, and eventually iPads. We evolved. And then we saw the light from the distant star. It took a while.