A battle in transwarp space

Transwarp space is the realm of subspace through which Borg transwarp conduits were formed. Normal limitations of subspace do not apply in transwarp space. Within transwarp space it is also easy to detect the residual and active transwarp signatures of other ships that generate conduits or had generated conduits. (TNG: "Descent"; VOY: "Dark Frontier", "Endgame")

According to Mr. Scott's Guide to the Enterprise (p. 112), the Federation transwarp drive submerged the ship into the empty parallel universe devoid of life, between the dimensional planes connected by the Tholian interphasic rift, seen in TOS: "The Tholian Web".
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