In mid 2374 Voyager received an encoded message from Starfleet via an alien communications system. The crew were unable to decode the message, and so it was eventually filed and more or less forgotten about.
At the end of the year Voyager encountered Arturis, an alien with an exceptional gift for languages who decoded the message for Janeway. The crew was directed to a set of co-ordinates, at which they discovered a Starfleet vessel named the USS Dauntless, NX 01-A. The message told them that the Dauntless had been launched on Stardate 51472, covering the 60,000 light year distance to Voyager in less than three months - an average speed of 240,000 times that of light. The ship was unmanned, and more than large enough to accommodate Voyagers entire crew.
On boarding the ship Voyagers crew found that it was powered by a "quantum slipstream drive" rather than a matter / antimatter reactor. The drive core was much more compact than a typical Starship's power core. It directed energy into both the two nacelles and the main deflector dish, with the latter system creating the slipstreaming effect. The slipstream is somewhat similar to the transwarp drive conduits used by the Borg, but can be generated by any Starfleet vessel with the appropriate modifications to its navigational deflector. Unfortunately, the quantum stresses on the hull are such that a normal vessel such as Voyager cannot survive in a slipstream for more than an hour or so.
It transpired that the Dauntless itself was actually a fake created by Arturis. His species had been assimilated by the Borg shortly after Janeway ended their war with Species 8472 and he blamed her and her crew for assisting the Borg during that conflict. He had created the Dauntless using particle synthesis, an advanced technology which appeared to function in a manner related to holodeck imagery. His plan was to lure the crew onto the ship and then send it into Borg space on autopilot so that the Borg would assimilate them all. Although Voyager's crew became suspicious of Arturis, he managed to abduct Janeway and Seven of Nine and escape into slipstream. Voyager followed and recovered its crew members, leaving Arturis to continue on to Borg space alone. Voyager managed to get 300 light years closer to home before its slipstream collapsed.1
Voyagers crew continued to work on the slipstream concept, and several months later managed to come up with a modified version of the technology. The new slipstream drive used a quantum matrix combined with benamite crystals and Borg technology to achieve far higher speeds than the Dauntless had managed, well in excess of a billion times the speed of light. Unfortunately, before the flight Lieutenant Paris discovered a previously unnoticed 0.42 phase variance in the slipstream threshold. Such a variance was more than enough to cause a catastrophic failure in the slipstream, leading to the destruction of the ship.
Captain Janeway nevertheless attempted the flight, sending the Delta Flyer ahead of the ship to map the slipstreams phase variations and send corrections back to the ship. The approach did not work, and Voyager was indeed thrown out of the slipstream after travelling only 10,000 light years closer to home, albeit without major damage. It was subsequently discovered that the phase corrections sent to the ship had in fact originated in an alternate timeline in which the ship had been destroyed during the flight. Surviving members of the crew had sent the information from decades in the future in order to rewrite history and save Voyager from destruction.
The benamite crystals used in the drive were highly unstable, and once they had been used up the flight could not be repeated. Voyagers crew continued to work on this impressive technology in the hope that further progress can be made.2, but the ship was ultimately able to reach Earth by other means.3