You did it right. It is totally true muons exist (they may even be responsible... for life, through cosmic rays). On the other hand, we highly need dark matter to explain all cosmological observations (however, alternatives exist), but dark matter still evade detection (the alternatives have drawbacks, potentially more severe, too). Therefore, the idea here is to combine both: dark matter that only interacts with muons, which can consequently explain why it is not been observed yet.