The swarm species was an unidentified territorial and hostile species which controlled a vast region of space in the Delta Quadrant by means of overwhelming any trespasser's ship by overwhelming it with a swarm of small ships.
According to Neelix, practically nothing was known about this species, including their name, their population, or what their culture was like. What was known was that most ships that crossed into their territory were never heard from again; some returned, but with the entire crew dead.
Their language was extremely complex and so unlike any language ever before encountered that translating it proved extremely difficult by using a standard Federation universal translator. The crew of Voyager never managed to translate more than the rough meaning of a few phrases. It consisted of a series of rapid chitters and clicks.
Although she knew that taking USS Voyager through swarm species' territory would be dangerous, Captain Janeway decided to do so in 2373, in hopes of avoiding circumnavigating their territory, which would have added over fifteen extra months to their journey back to the Alpha Quadrant.
During Voyager's attempt to cross their territory, it was determined that this species protected their border with sensor net composed of nearly undetectable tachyon field, which was composed a series of interlaced tachyon beams. Any ship that crossed the field, was immediately confronted and warned to leave, or worse.
In the case of the first confrontation with a shuttle from Voyager, two members of the species transported aboard the shuttle and attacked Tom Paris and B'Elanna Torres with what was later described by The Doctor as "some type of neuro-electric weapon." This encounter also marked Voyager's first encounter with a species that possessed transporter technology in the Delta Quadrant.
Those that did not heed their warning were attacked. Their vessels were shuttle-sized, but, when working in large groups, could disable a bigger ship by swarming it, landing on its hull, draining power from it, and finally transporting a boarding party aboard.
During Voyager's encounter with the species, Janeway decided to ignore the alien's warning, and attempt to slip through their territory, undetected. It was thought that by firing a refraction pulse while adjusting the ship's shields to match, would cause the ship to refract the sensor net's tachyon beams around the ship without making it appear as if the net had been disrupted.
Upon reaching the border, they detected what appeared to be a single massive ship, but upon further investigation, it was determined to actually be a mass of what looked like thousands of idle ships. Once Voyager entered the territory, and attempted to engage maximum warp, they discovered an unanticipated drag on the engines. According to sensors, they encountered a resonance particle wave, which dampened Voyager's warp field, causing it to lose speed.
Soon thereafter, they discovered a wrecked and listing Mislenite freighter, with a single survivor aboard. The survivor described the attack, stating, "There were thousands of them, little ships, swarmed around my freighter, covered every square meter of the hull. Then they began extracting energy from all the ship's systems."
Unbeknownst to Voyager, a single ship from the swarm had remained attached to the freighter's hull. When it revealed itself, it fired a polaron burst, which changed Voyager's shield polarity, and made it visible to the rest of the swarm.
When the swarm reached Voyager, each of the scores of ships emitted an interferometric pulse, which were modulated in opposition to Voyager's shield frequencies, causing them to be completely cancelled out. Indeed, the same pulse also reflected Voyager's attempt to fire phasers as well.
With Voyager defenseless, the swarm ships began clamping onto the hull and began draining Voyager systems. As this began, Harry Kim determined the pattern used with the interferometric pulses: "a lattice, connecting all the ships to each other." Captain Janeway was able to turn the pulses back on the swarm ships, and after using a shield remodulation to inverse the pulse harmonics, they were able to fire phasers at one swarm ship, destroying it and causing a chain reaction within the swarm, causing them to disband and allow Voyager to leave their territory. (VOY: "The Swarm")