Your husband committed violent assault and his potential cheating is what you want to focus on?
Would a man attack his wife for his cell phone if he wasnâ€™t cheating?
Don't ask random strangers questions we cannot possibly answer. The only person who knows that answer is your husband. And maybe a PI who can investigate whether your husband is having an affair with someone else.
One might say, the act of you looking into his phone is a nearly unforgivable betrayal of the trust that makes up your marriage. That's another way to interpret his overblown reaction towards him finding out your snooping activity, (an explanation not a condoning of said violent response).