ETI and dark matter

A very wild though came to me, so please to kill me if I'm talking nonsense.

What if ETIs found a way to use the energy of a star for their own benefit,
and that is why we don't see a good portion of the matter in the Galaxy ?

For example, they could have formed Dyson spheres (solved the instability
problem) around stars and thus absorb all the star light. The only evidence
of the star would be in deep infrared (radiation of the Dyson sphere's
temperature).

If done on truely gallactic scale, then intelligent life could be
responsible for the missing 40% of matter in the Galaxy, which we currently
address to (so far theoretical) "dark matter"....

OK. Shoot me.

Rob