The "Silver Blood" was a biomimetic lifeform from a Y-class "demon" planet in the Delta Quadrant. Appearing as metallic liquid in its natural state, this lifeform had the ability to change itself into that with which it came into physical contact, be it biological or technological.
In its original state, the silver blood appeared as puddles of a highly viscous compound packed with liquefied deuterium. It had organic properties, such as containing protein molecules. Its temperature appeared to be only 12 degrees, even though its surroundings were almost five hundred degrees kelvin. Apart from deuterium, the lifeform also contained hydrogen sulfate and dichromates. (VOY: "Demon")
The Silver Blood was encountered by the crew of the USS Voyager in 2374. While attempting to sample it after determining it contained deuterium, Tom Paris and Harry Kim came into contact with the substance and were duplicated down to their memories, Starfleet uniforms and combadges. Upon duplicating the Humans, the Silver Blood became sentient for the first time and quickly hungered for more. Holding Voyager physically hostage on the surface of its homeworld, the Silver Blood copy of Kim negotiated to have Captain Kathryn Janeway give over samples of her crew's DNA so that they could be copied. She agreed and Voyager was allowed to leave with a supply of deuterium. (VOY: "Demon")
By 2375, the Silver Blood had replicated Voyager itself, including its computer memory and programs, as well as The Doctor. By this time, the crew was able to breathe oxygen and had forgotten that they were copies of the original Voyager crew. Traveling across the Delta Quadrant while trying to return to Earth, which they thought was home, the crew experienced such encounters as the N'Kree attempting to conscript Voyager into their battle fleet or the Kmada attempting to sabotage their life support systems with low-frequency theta radiation.
At some point during their travels, the ship began to utilize an enhanced warp drive, which would allow them to reach Earth in just two years - the original Voyager would still have needed almost six decades to get there on its own. While the effect of this new warp core had no harmful side-effects on cellular life, it did, however, cause all people and objects made of the Silver Blood to eventually break down and die, first striking the engineering staff, and subsequently spreading.
Having discovered their true nature after determining that the degradation only affected objects that had been on the ship before their visit to the demon planet – as well as testing the body of the recently-deceased B'Elanna Torres duplicate – the crew adjusted the environmental controls to mimic a class-Y atmosphere in the hope of slowing their degradation. Although the Janeway duplicate was still resolved to continue their remembered journey to Earth, some of the crew questioned her decision to continue on to Earth; the Tom Paris duplicate was particularly bitter about it, immediately dismissing himself and Torres as fake despite their recent marriage, questioning the Harry Kim duplicate on what would happen if he returned to Earth to find the original Kim present, and wondering why they should even bother obeying the captain since she had no "real" rank. Aside from returning to a class-Y planet, The Doctor's only other suggestion for a cure was to find the original Voyager and take samples to stabilize their own condition, but Janeway dismissed that as impractical as they had no way of knowing where the original ship was in relation to them or if it even still existed.
An attempt to repair the ship by landing on another class-Y planet failed when they were attacked by a ship representing the Ord'mirit Mining Treaty, with Voyager too damaged to simply destroy the ship's weapons and Janeway reluctant to destroy another ship due to a misunderstanding. Despite her desire to get the crew back to Earth based on her own memories of their journey, the death of Chakotay during an argument forced Janeway to recognize that they couldn't continue in this manner, prompting her to order the ship to turn around so that they could reach the demon planet.
As time went on, the ship's degradation increased as they progressed despite their use of Borg nanoprobes to try and stabilize the ship; by the time that they were five weeks from the demon planet, the holoemitters had disintegrated to the point that The Doctor's program was lost, and the senior staff were down to Janeway, Kim, Neelix, and Seven of Nine, with Tom Paris incapacitated. Shortly before her death, Janeway suggested the creation of a log buoy from non-Silver Blood components detailing their history for others to find, but it was destroyed in the torpedo tube before it could be launched. With only ten hours of life-support left as they detected the approach of the original USS Voyager, the ship and remaining crew literally fell apart as Kim tried to slow them down by ordering the ejection of the warp core after all other controls were too damaged to continue; all hands were lost and the real Voyager never knew of the fate or even the existence of its counterpart, its discovery simply being a brief recording of an unknown tragedy. (VOY: "Course: Oblivion")
Harper and baby
In the Voyager relaunch novel A Pocket Full of Lies, the crew of Voyager briefly speculate that a second Kathryn Janeway encountered in the novel is the Silver Blood duplicate, but this theory is dismissed as the crew still believe that the Silver Blood duplicates were unable to leave the Y-class planet's atmosphere (the other Janeway is later revealed to have been created by a Krenim attack on Voyager during the events of "Shattered").