“[Co-writer Adam McKay and I] were both really jazzed about the idea of, in the third act, in a movie in which we will have seen shrinking a bunch, let’s take it even further in the third act and introduce what, in the comics, was the microverse, in what we call the quantum realm.”
In comics, the Microverse is a parallel universe that can be reached by shrinking down to sub-atomic size. Numerous Marvel characters have traveled to the Microverse, including Hank Pym and Scott Lang, but also Captain America, the Fantastic Four, and even the Hulk. The Microverse was also the setting for the cult-classic licensed series Micronauts.
Finally, Reed also mentioned the film's name-check of Tales To Astonish, the title in which Ant-Man made him debut in the '60's, a detail that reportedly survived Wright and Cornish's draft, saying “I know Edgar and Joe had had a nod to Tales to Astonish, but that was something I absolutely wanted to preserve. I loved it.”