The mote in God's eye

In the year 3016, the Second Empire of Man spans hundreds of star systems, thanks to the faster-than-light Alderson Drive. No other intelligent beings have ever been encountered, not until a light sail probe enters a human system carrying a dead alien. The probe is traced to the Mote, an isolated star in a thick dust cloud, and an expedition is dispatched. In the Mote the humans find an ancient civilization--at least one million years old--that has always been bottled up in their cloistered solar system for lack of a star drive. The Moties are welcoming and kind, yet rather evasive about certain aspects of their society. It seems the Moties have a dark problem, one they've been unable to solve in over a million years.