The average temperature of the universe is 2.73 K based on the Cosmic microwave background.
What (if any) are the predicted effects of dark matter on this temperature if it is found to be true ?
The dark matter has no effect at all on the microwave radiation passing through it. If it did, it wouldn’t be dark. It has, however, had a very significant effect on the evolution of the Universe, which in turn has left a number of signatures in the CMB.
Why do I feel like I’ve already answered this question?
I must be missing something about the OP’s question.
The temperature is a measurement. It doesn’t matter if we know what makes up that measurement. It already includes everything measureable. Why should identifying dark matter change the measurement? For that matter, how could it possibly change the measurement?