Focusing on truth and reality:
* Fact
* Truth
* Reality
* Verity
* Certainty
* Proof
* Evidence
Focusing on lack of imagination:
* Reality
* Truth
* Fact
* Logic
* Reason
* Rationality
Focusing on historical accuracy:
* History
* Authenticity
* Accuracy
* Documentation
* Record
Other possibilities:
* Truth (general, encompassing all above meanings)
* Verifiable
* Provable
* Authentic
* Genuine
* Real
The best antonym will depend on the specific context in which you are using the word "myth." For example, if you are talking about a mythical creature, an antonym might be "real animal."