[wizzwizz4]

> 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.)

[mcbits]

Maybe stars are the Dyson spheres, and their emissions are the waste energy that wasn't worth harvesting.

[wizzwizz4]

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.