The Long Night by Poul Anderson

The legendary Nicholas van Rijn had, of course, been right all along. Just as he had foreseen, the Polesotechnic League – that great empire of merchant princes – had flowered and then crumbled into the vastness of space. The same fate would befall the Terran Empire that succeeded it. Even heroes like Dominic Flandry lived under the shadow of their eventual extinction – the ever-hungry darkness that would take him and his world in the end. But for those who came after, those wretched heirs of Terran civilisation, the darkness was no impending tragedy. This time they were facing the reality of THE LONG NIGHT