[neuronic]

It's not dark matter because it blocks light. It's dark matter because its only observable interaction is by gravity.

So aside from being perfect in terms of energy capture so that the underlying stars emit absolutely no EM at any frequency, the proposed Dyson spheres would also need to be completely transparent to any background EM such as visible light picked up by sensors like Hubble.

[m4rtink]

Good point - if we are able to detect gas clouds, we would be likely able to detect such swarms as well simply by all the blocked light.