While the Ledosians were a highly-advanced warp-capable species, the Ventu were a nomadic and simplistic people. Centuries ago, when aliens known only by their Borg designation, Species 312, encountered their world, they constructed an energy barrier separating the still primitive Ventu from the rest of their society.
Ledos had very strict rules governing starship pilots. Pilots deemed reckless could face consequences including a mandatory three day piloting course, or the revocation of their right to fly in Ledosian space.
In 2378, Ledos was visited by the USS Voyager, where the crew conducted shore leave and attended a conference. When a shuttle became trapped in the energy barrier, Seven of Nine and Chakotay were trapped within it. They interacted with the Ventu, who assist in treating their wounds.
Voyager attempted to bring down the barrier using its phasers in order to retrieve their crew, only to find it was already being brought down from the inside by Seven of Nine.
The Ledosions were extremely grateful, and began sending research teams to study the Ventu. Realizing that the cultural contamination could destroy the Ventu society, Voyager re-initialized the barrier, and promptly left Ledos. (VOY: "Natural Law")