... wondered if most of the universe is hidden away in dark "matrioshka brain" like objects. Could be Dyson spheres....
Both the Dyson sphere and matrioshka brain radiate with the same luminosity as the star they draw energy from. The luminosity is low temperature emission.
Maybe something like this: ll pegasi
Sure, the luminosity is the same, but considering the re-emitted flux is much lower due to the bigger surface area, wouldn't a star with a partial Dyson Swarm/Matrioshka Brain/... appear to us as a smaller and/or colder star that is somehow much heavier than it seems to be based on its apparent luminosity? Isn't the star's brightness our main indicator of its size? So how would we know our formula to calculate their size is influenced by such phenomena unless we see large differences between the sizes we think they are and the gravity they appear to have? (which happens to be the case)
A good example is four 15 watt florescent bulbs, a 60 watt incandescent bulb, and an electric heater using 60 watts. Normal human reaction is to say that the florescent bulbs are much "brighter" and that the electric heater is not a light. The bolometric luminosity is the same for each item (assuming we ignore convection or add watts to compensate). The visual luminosity of the florescent is much higher. In parts of the infrared the incandescent has highest luminosity. In microwave frequency the heater is much "brighter".
So how would we know wether we're looking at a red (super)giant or a main sequence star of the same luminosity surrounded by a
Dyson swarm approaching the density of a full Dyson sphere?
My instinct is that variable coverage is more likely in a partial sphere. As coverage varies from 1% to 20% the visual part of the star's spectrum would vary between 80 and 99% visual luminosity. The far infra-red spectrum would also vary but not necessarily synchronized with the visual variability. The radiators could be on multiple surfaces. Is possible that most would radiate heat away from the star so far infra-red would vary inverse to visual.
A partial dyson sphere acting as a Shkadov thruster would have less infra red (compared to stationary) if it was moving towards us or perpendicular. If they are moving away from us the infra red luminosity would be much higher. Mirrors could create strange stars with higher visual luminosity too but doubling is unlikely.
Orbiting mirrors and radiators also red and blue shift as they move.
All stars are a point of light in our telescopes.
As far as measuring diameters is concerned, all stars are badly resolved blobs of light surrounded by diffraction patterns in our telescopes.
Incomplete Dyson swarms with individual elements between the size of NYC and Luna, assuming there's an upper limit to their practical size, would not be detectable by our current or upcoming telescopes. We would assume the star is just somewhat more variable. The movement and reflected radiation of the individual elements is not enough to be detectable as an individual space station/planetary body. The scattering effect they have on the radiation emitted by the star might be actively filtered away to resolve the star better.
And all this still assumes that ETI somehow doesn't come to the conclusion that at a certain scale, rather than using the radiation emitted by natural fusion reactors, it's far more practical to just do the fusion in not-so-natural fusion reactors and become independent of stars in the first place.
With 84% of all matter unaccounted for by our measurement of star sizes, and our telescopes having a limited capability to pick up such artificial fusion reactors emitting about as much energy as a brown dwarf, there's plenty of room to hide in plain sight.