The Galactic Barrier

Fearing retaliation from Species 10-C, Rillak moves up the timeline for first contact. She, T'Rina, and several diplomats board Discovery, which jumps to the galactic barrier and begins the dangerous flight through it. Before they lose contact with the Federation, they are contacted by Vance, who tells them that the DMA is now close to destroying Earth and Ni'Var. Meanwhile, Book and Tarka need programmable anti-matter to make it through the barrier themselves and travel to a stash of it that Tarka has in an abandoned prison where he was once held. Tarka explains that he had befriended a fellow prisoner, Oros, who built a machine to transport them to a peaceful parallel universe. They were separated and Tarka does not know if Oros was successful, but he hopes so and has built his own machine so that he can also travel there.

Summary from Wikipedia, CC BY-SA 4.0.