A expanse is simply a large area with some kind of characteristic. Truthfully, I not sure what you're trying to say. There also the consideration that the expanse wasn't a permanent feature, it was growing in size, so working backwards there would have been a time in the history of this part of the galaxy when the expanse was tiny. It would have been possible to easily travel around it. The Cardassian poet could have gotten to Vulcan by traveling through what would one day be the expanse. Prior to the construction of the first of the spheres there would have been no expanse at all.