Dark matters. Book 3 Shadow of heaven - Christie Golden

"What if earth be but the shadow of heaven, and therein things each to other like more than on earth is thought?"
- John Milton, Paradise Lost
Matter from the shadow universe is appearing in our own: matter from our universe is appearing there. The universes are unravelling, and to halt the process of destruction the crew of the USS Voyager - minus their First Officer and their pilot - must gather ever piece of the deadly material they've been unwittingly trailing in their wake. In the shadow universe where they are stranded, Chakotay and Paris must do the same. Harry Kim has become entranced by a girl from the shadow reality, but she will have to return there - when they've completed their quest. The Romulans realise the threat they have unleashed and join forces with Voyager to repel it, enabling popular "guest character" Telek R'Mor to lay a crucial role. At the eleventh hour one final piece remains...but Voyager cannot find it. Someone from their past is the only one who can help them. And fortunately Kes, who left the ship when she evolved into a higher lifeform, has always been the helpful sort...