My mom saw my dad's messages with another woman. She showed me these messages and told me that my dad has been talking to this woman since years ago. She also told me to take pictures of the messages, so I did. My dad came home and saw that a new message from the woman has been read. He got mad at mom and told her not to touch his phone, I don't know if I should act normal or not since he doesn't know that I know about it as well.

Personally I  would wait. This sounds more like a marriage issue between two partners. I don't really know how you could solve the situation by getting involved, but you should wait and see what kind of agreement your parents decide to make. If the entire thing escalates quickly (if the discussions are getting worse) then you should get involved since it's unavoidable to pretend not knowing if you they yell at each other. But if everything settle down and they don't mention about it that means that they came to an agreement or the mother accepted his apology or something like that. Only time can tell what would happen. It must be really traumatic to experience that so I hope your family get better 🙏

CLC's reply is awfully wrong. It is likely that OP is a minor. You should avoid getting involved unless the situation escalates to the point of violence, in which case you could call 911. Taking sides can be dangerous. The fact that your mother involves a child in marriage problems is a sign of irresponsible parenting, which is unfortunate. I am sorry that your dad acts that way, but there may not be much you can do about it. Men are shit and you'll learn it through time. For the record, I'm not saying your mom is satan for involving you on this, she is probably helpless and feels lonely. You should definitely help/support her in every other way. But as for this, you are not in place to be a "mediator". Your parents are adults and they should sort things out themselves. This is not something a child should worry about.

To Azima Tempest, Simple don't be used by your mom or by anyone else. You don't need to be someone's pawn yet so live your life the way you want to. If you mom says "hey I need you to get more pictures of your fathers text messages off his phone" You say to her "No mother I'm not going to do that because it's not my place to spy on someone's relationships, so Call the Phone Company" so Simple. But If you mother says that you have to because she is the parent then Tell your father on what she is doing because if a Parent Uses their child either as Bait or is using them like a pawn then they are not Fit to be called a parent and their a piece of shit for doing so.