[maxander]

Indeed, Dyson spheres would need to release some amount of waste heat, due to the laws of thermodynamics. If there were too many of them that heat would be visible (at least to infrared telescopes,) so we know that’s not the explanation for dark matter. Same with dust and other non-luminescent matter- it’s all way more visible than dark matter has to be, somehow.

[freemint]

What prevents that waste head being as warm ad the peak in cosmic background background radiation

[ncmncm]

Literally every last thing in the whole universe that can radiate heat is at least as warm as the cosmic background radiation. You cannot shed any heat at all unless you are warmer than the background radiation, because you would be absorbing as much heat from it as you radiate. If you were colder, you would necessarily be absorbing more.

Thermodynamics does not play around.