Travelers
But is it that simple? If Starfleet is tampering with the nebula, the obvious conclusion is that they are doing it with the goal of sending us all home. That isn't so alarming to me, they're having to use resources in order to ensure our survival, so it'd reason that they would want to stop it. At the very least, they would want to understand it.
I've been on the Enterprise for over a year and since I arrived the crew of this ship has been studying the phenomenon that brought us here. It's a group of people far smarter than I. Clearly, they've made little progress despite their best efforts and that of some of the travelers. So sending us home isn't new, nor is it that easy.