(covers information from several alternate timelines)
A subspace vacuole was a type of dimensional distortion which can act as a means of transit between dimensions. Subspace vacuoles can also pose a hazard to navigation, due to the fact that contact with a vacuole while traveling at warp speed could lead to the destruction of one's ship.
In 2371, Harry Kim was pulled into a subspace vacuole while the USS Voyager was investigating a new element they discovered in the ring system of a class D planet in the Delta Quadrant. Arriving in another dimension, he found that a species called the Vhnori utilized these vacuoles, which occurred naturally all over their homeworld, as passageways to what they called "the Next Emanation," which they believed to be the afterlife. Calling the vacuoles "spectral ruptures," they believed that a person, once dead, revived on the other side, and was reunited with their loved ones. Eventually, Kim was able to use another vacuole to return to his own dimension by using a cenotaph. (VOY: "Emanations")
Later that year, Voyager entered a dense region of subspace vacuoles, while attempting to avoid the Vidiians. In an effort to avoid being ambushed, the crew decided to program the auto-navigation system to allow them to traverse the area at warp, dropping to impulse only to make course corrections. Due to the future version of Kes, Voyager ultimately was forced to fight against a Vidiian starship in an alternate timeline, but successfully repelled the attack and prevented the version of Kes from the present from transforming into her counterpart from the future. (VOY: "Fury")
In 2375, Voyager's operations officer, Harry Kim, and Derran Tal, a resident of the Varro generational ship with whom Kim had become romantically involved, took one of Voyager's shuttles to explore a class 3 nebula, where they discovered a subspace vacuole at the center of the cloud which acted as a gravitational anchor. (VOY: "The Disease")