Huge hugs OP.
My ex had no idea that when logged onto a Gmail account and using the Chrome browser, the default (automatic) setting is to save the (abbreviated) history in the Google Account. This is stored on a server somewhere in the world, probably the States. And it has to be manually deactivated in those settings. However, it does not show every single page visited. It shows kind of an abridged version. But that was enough for me.
Most people have no idea it even does that.
Your husband maybe just figured it out recently.
Better to know someone is rotten to the core than not. The lies, for me, the fakery of the relationship, the lies told to my face when I held evidence in my hand, shocked me more than what he was capable of doing.
His second ex wife had some odd public FB posts about narcissism and abuse, but I always believed his version of events. I even wanted to believe his storied about the Google history, I thought I loved him so much. It was all an illusion and THAT was the hardest part.
I think these men are mentally ill.
I hope you can get legal advice.. and yes, do a backup of all that stuff.. set up a separate email that HE has no idea about and stash it away there. When you eventually tell him that you know, he may freak out. It might be better to do it in a public place, just in case. You don't know him really..... I realised I never really knew my ex.
He didn't get violent, he just tried to twist it all and blame friends, technology accidents... a whole load of crap.
Drugs, prostitutes, cheating and stolen goods. He was not the person he presented to me. But THANK GOD I found out... took a long time to heal though.... I wish you strength.