Multiverse
The multiverse was the set of parallel universes, which included the prime universe and the Mirror universe, as well as theoretically many others. On the physics level, matter from different universes differed by having a different quantum signature. ()

Several methods of crossing from one universe to another existed, including traveling along the mycelial network, certain transporter accidents, and interphasic rifts.

The mycelial network was believed to underpin all quantum realities, extending across the entire multiverse. This proved to be a threat when  of the Mirror universe built a reactor which corrupted it, which threatened to destroy the network in all realities, and with it all life. ()

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Physics