Formal:
* Verity: A truth, especially a fundamental principle.
* Fact: Something that is known or proven to be true.
* Reality: The state of things as they actually exist.
* Authenticity: The quality of being genuine or real.
* Veracity: The quality of being truthful or honest.
Informal:
* The real deal: Something genuine and authentic.
* The truth of the matter: The real situation.
* The facts of the case: The actual details of a situation.
* The lowdown: The real story or information.
* The nitty-gritty: The essential details.
More nuanced:
* Accuracy: The quality of being correct or precise.
* Validity: The quality of being logically sound or well-founded.
* Integrity: The quality of being honest and having strong moral principles.
* Honesty: The quality of being truthful and sincere.
Literary:
* Veridical: True or accurate.
* True-blue: Completely honest and reliable.
* Unvarnished truth: The truth without any embellishment or hiding.
The best word to use will depend on the specific context of your writing.