Many centuries prior to the 2370s, an advanced alien race known as Species 312 discovered that a majority of the population that was technologically advancing was polluting the area of a more primitive species that was their genetic ancestors, the Ventu.
The aliens placed an energy barrier to act as a shield around the Ventu area, protecting them from the Ledosians. For centuries, the Ledosians had tried everything to penetrate the field without success.
In 2378, the USS Voyager made first contact with the Ledosians and set in for a two-day leave. When Seven of Nine asked to attend a three-day conference, they decided to stay and extend shore leave to the whole crew. However, her and Chakotay's shuttlecraft crashed within the barrier. They were able to neutralize it with the help of the Ventu, but the Voyager crew restored the barrier when it was feared that interaction with the Ledosians would destroy the Ventu's way of life.
The Ledosians' expedition force was able to scan Seven's deflector modifications, so the Ledosians could use the scan data to make a copy of the deflector modifications to reopen the barrier themselves in due time, according to Seven's concerns. (VOY: "Natural Law")