A Black Hole

Travel was always tedious, but humanity would rather keep traveling tediously, because that meant no civilization had discovered their route, and they were safe.

Upon reaching the Perseus Arm, the density of stars gradually increased. However, since they were still near the edge of the spiral arm, it was not overly dense. The average distance between stars was roughly seventy light-years. For a route crossing the spiral arm, that was already close enough, and it meant humanity did not need to worry about resources.

With Level 4 Civilization technology, as long as there was a Star System, energy could be obtained. Even without a Rocky Planet, enough light elements such as Hydrogen-Helium-Hydrogen could be gathered from Gaseous Planets. Those could then be used in light-nuclei fusion to generate energy, which would power reactors that separated hydrogen molecules to obtain antimatter.

Of course, without Rocky Planets, heavy metal elements could not be acquired. But the universe was so vast, and the Perseus Arm contained so many stars, that humanity had no need to worry. If one system did not have what they needed, they could simply continue to the next Star System. After all, no journey would completely exhaust their fuel.

Faced with stopping points averaging seventy light-years apart, humanity did not intend to travel from star to star. Instead, every voyage crossed at least three hundred light-years. A fully fueled human fleet could travel five hundred light-years in one go. Limiting each trip to a little over three hundred light-years was mainly to conserve fuel for possible emergencies, while avoiding seventy-light-year hops one by one was for concealment and efficiency.

Thus, the human Great Fleet traveled along the edge of the Perseus Arm, year after year.

Then, at some point, the Lyra received a message from a Probe Team. They