Wouldn’t dark matter need to make up exactly 50% of the universes mass for the mirror matter hypothesis to be credible?
Yet dark matter makes up 96% of the Universes mass so this does not make sense. 4% and 96% seem no where near equal.
Your argument would be correct if symmetry was preserved. However if mirror symmetry is broken then the mirror matter could be either heavier or lighter than ordinary matter and would interact only weakly if not at all via the electromagnetic, strong and weak forces and only via gravity. Hence if mirror matter existed it would be by definition be dark matter.
The 96% includes dark matter and dark energy which may or may not be coupled. The dark matter is about 6 times the ordinary matter we interact with (we see less than 25% of that). These numbers may become important numbers if mirror matter can be shown to exist or string theory will ever be confirmed.