SPOILERS for Venom ahead.
Venom director Ruben Fleischer says an early iteration of the film's opening took place on the symbiotes' home planet, Klyntar. The version of the movie that was released in theaters instead begins in outer space, as one of the Life Foundation's space crafts comes crashing down into earth's atmosphere - having acquired (and, thusly, been inadvertently sabotaged by) a pack of symbiotes from a comet that the ship's crew passed by over the course of their expedition.
For the most part, the theatrical cut of Venom is a superhero (check that, super-antihero) origin story for Tom Hardy's Eddie Brock and focuses on showing how the human reporter encounters and subsequently bonds with the symbiote Venom. At the same time, the movie includes direct references to Venom's backstory and the world that he and his fellow symbiotes call home. For example, there's a moment where Venom tells Eddie that he relates to his human host because the character was something of a "loser" on his own native planet.
Beyond tongue in cheek nods like that, admittedly, Venom doesn't reveal a whole lot about the symbiotes or where they come from. In an interview with Cinemablend to promote the movie, Fleischer explained that this was very much intentional and talked about one of the early versions of the film's opening scene:
The beginning of the movie was a hard one. Like, how much do we want to tell the backstory? There was a version [of the opening] where there was a planet crawling with tons of symbiotes that were collected and taken back to Earth by the Life Foundation. They had encountered them. But it kind of felt like, my instinct was it would be better to keep it more mysterious and just know that they've retrieved something from space. We don't know exactly by what means or how. Jenny [Slate]'s character later says they encountered it on a comet that was passing, and they retrieved these samples and brought it back to Earth. But yeah, I thought it was better just leave it a little bit mysterious, the backstory of the aliens.
In a separate interview with Cinemablend, Venom producer Matt Tolmach backed what Fleischer said and revealed that the film's production team had even drawn up concept art for Klyntar. Ultimately, however, the decision was made to leave that aspect of the Venom mythology for a sequel to explore:
We toyed with some art work that we loved... We toyed with it. But we wanted the movie - it's out there. It's part of our mythology. It's part of our backstory, Klyntar and where this all came from and who knows what's next. But, it made sense to, in this movie, to let the origin of those things kind of unravel in the storytelling is as opposed to going [to that planet].
All things considered, the decision to leave the symbiotes' origins a mystery in Venom was probably a smart call. The film had enough on its plate to do already, between setting up Eddie Brock's backstory, introducing the Life Foundation and their evil goals, and establishing important facts about the symbiotes' biology and physiology. While Venom 2 is all but guaranteed to happen now thanks to the first movie's record-breaking box office opening weekend, it probably won't delve fully into Klyntar's history either. Rather, the sequel is expected to reveal more about the symbiotes and their... peculiar abilities, while at the same time keeping the action based on earth.
Still, who knows - after Venom and Eddie are done fighting Carnage and Cletus Kasady (Woody Harrelson) in the sequel, perhaps the pair will make a trip to outer space to visit Venom's home world in Venom 3.