> We have only recently put a man on the moon, and now we're saying we would be able to detect dyson spheres...?
A moth doesn't even have electricity, but it can detect a lampshade. A Dyson sphere is basically a stellar lampshade; we can detect it if the laws of thermodynamics hold, and we're pretty sure they do. (A Dyson swarm is similar, though a bit harder to detect.)
The spectral signature would be different if that happened. If all of the stars are Dyson spheres, then… maybe our models about stars are fundamentally wrong. But I doubt it; the maths to work out what the signature should be (given the composition) is pretty simple. Take a blackbody, put spectral lines as measured in the lab onto it, then redshift it.
A moth doesn't even have electricity, but it can detect a lampshade. A Dyson sphere is basically a stellar lampshade; we can detect it if the laws of thermodynamics hold, and we're pretty sure they do. (A Dyson swarm is similar, though a bit harder to detect.)