Asking for what pseudoscience might explain a storytelling need:
Given parallel dimensions (worlds), we discover that through a portal device, a person equipped with a special spacesuit-like outfit can temporarily inhabit an "adjacent" world. If (when) their power supply is depleted, they "snap back" to the portal, but otherwise can interact with the other dimension freely. There is supposed to be some sort of "navigation" inherent in the portal itself (à la : "dialing a Stargate")
This includes breathing the air and obtaining power, potentially eating food, etc. Ie: they do not have to stay completely isolated from the environment as "spacesuit" might suggest.
The origin of the portal and suit technology is extraterrestrial and doesn't need a detailed explanation. However, some scientists on Earth will naturally be investigating its workings. (The human suits were manufactured by humans, but based on captured alien widgets, with about as much understanding as asking a 1923 radio tinkerer to build headphones for your Android phone.)
I've a notion to attribute this all to "string theory" and avoid a detailed analysis, but wonder if there's any specific or plausible mechanism that could work like this? Eg, for some throw away lines from the scientists.
(NB: there's no reason to believe — for nor against — that the alternate world couldn't be some far-distant place in our own universe, ie, someplace one might eventually be able to fly to, in infinite time)