I didn't install anything, Kurt.
I should have been more clear. This type of garbage comes along with items you may have installed that carried adware with it. This is very common from software aggregate sites such as softonic.com, downloads.com, and others.
MalwareBytes doesn't find everything since there's always something new, and they need to first know about it, then add those definitions to their database.
You many not have anything installed, but is simply where you visit on the Internet. Many, many sites happily host ad servers to make money. They don't choose the ads the site displays, the ad server company does. And the losers who develop MacKeeper buy LOTS of ad time.
It may be as simple as installing an ad blocker. I use the free version of AdGuard. It's available in the App Store.
If you use Chrome, it's difficult (or it used to be) for MalwareBytes to scan Chrome's user installed extensions. Check those. If there are any you don't recognize, disable and delete them. Same for Safari. Open its preferences and look to see what extensions are there.