Cloak and Dagger

Dark Matters, Book 1: Cloak and Dagger.

Allying with a mysterious and powerful alien species, the Romulan Empire from 20 years in the past has created a new wormhole – one leading straight to the present-day Delta Quadrant and the U.S.S. Voyager. There a sinister plan is set in motion, as the Romulans wish to gain a head start on future Federation technology by capturing Voyager and manipulating the phenomenon known as "dark matter"... with potentially disastrous consequences for the entire cosmos.