How can you discover a Dyson sphere without actually going there? By definition a Dyson sphere captures all light put out by the star it encloses. Therefore it can only be illuminated by light from distant stars - against the star field a Dyson sphere is going to be incredibly dark.
For that matter, how are we going to discover a Ringworld? Surely the puppeteers that built it will coat it in some kind of stealth technology. Matt black paint for example.![]()
As well as the 400 billion "known" stars, we've already had it confirmed that there are "dark" systems out there that will need to be discovered even before they're explored. A Dyson sphere is just one possible dark system.
The Puppeteers didn't build the Ringworld. There's a theory it was built by the Pak but no one really knows, with the possible exception of Larry Niven.