What happened to the Dark Universe?
Ah, Dark Universe, we hardly knew you. The Dark Universe actually started not with The Mummy, but with 2014's Dracula Untold, which starred Luke Evans as Vlad Dracula, and tried very hard to marry a very creative version of "history" with Bram Stoker's vampire character, to pretty much no success. It was meant to be an origin story for the character, rather than sticking to the familiar Dracula tale, and includes a scene of Dracula and Charles Dance's evil vampire Caligula in the present day at the very end, as if setting the characters up for more. If The Mummy had done well, Evans' Dracula would likely have appeared in a following film.
Universal was banking on a lot, casting sequels and teasing more monsters to come even before The Mummy had come out -- they even released a "cast photo" of Tom Cruise, Sofia Boutella, Russell Crowe, Johnny Depp, and Javier Bardem, who had just been announced to play Frankenstein's monster in the Bride of Frankenstein movie. (This is the only time Bardem and Depp have appeared physically in anything related to the Dark Universe.) But, seeing the response to The Mummy and unable to lock down anyone for their following movies, Universal quietly canned its shared monster movie series.