Invisible alien researchers who secretly ran harmful experiments on Voyager's crew until Seven of Nine and Captain Janeway exposed them.
The Srivani are a humanoid species who carry out scientific research on other lifeforms. They use a cloaking technology that keeps them out of phase with normal space, letting them observe and manipulate their subjects without being seen, and they show little concern for the harm they cause.
In the episode Scientific Method, crew members including Chakotay and Captain Janeway suffered sudden ageing, stress and other strange effects. Seven of Nine, whose ocular implant allowed her to detect the hidden observers, found that the crew were the subjects of Srivani experiments. Janeway forced them to leave by piloting Voyager on a dangerous course between two stars, threatening to destroy the ship and their research.