Hello, welcome to a Disney Theory! (This might be the only one I do, but if you like this, then request me to do more)
Today, we're discussing the portals / realm in Big Hero 6. They were originally used to teleport things (like Callaghan's daughter!), but they lead to another realm. That reminds me of the Negative Zone...
The Negative Zone first appeared in Fantastic Four Vol 1 Issue #51 (June 1966). The Negative Zone is an antimatter universe which parallel to Earth. It's ruled by competing warlords, most notably Annihilus and Blastaar. You might recall it as the alternate world in the new Fantastic Four reboot.
So, now some points for why it could be the Negative Zone
#1: Stuff can be teleported through the Negative Zone and the Realm
In the Ultimate Fantastic Four and the new reboot coming this year, the government began to teleport things through the Negative Zone, where they would come back to us (which is the same as the Realm).
#2: Things (Marvel puns are funny) go wrong
In the Ultimate Fantastic Four and the new reboot coming this year, after teleporting some small things, it goes okay. But then they send humans, and some things go wrong. In Big Hero 6, it results in Abigail being trapped. Though in Ultimate Fantastic Four, it gives the team their powers.
#3: Disney/Marvel doesn't own the film rights to anything part of the Negative Zone, Fox does
Marvel (in the comics) has been killing various heroes that Fox owns (since Sony partially caved to Marvel/Disney's) bidding, it only makes sense that Marvel would build a new realm to have their own Negative Zone.
Well, thank you for reading, and this has been a Disney Theory!