The machine planet was a planet located in an unknown area of space, home to a species of sentient machines. When the NASA Voyager 6 probe fell into a black hole in the early-21st century, it reemerged in this planet's local vicinity, where it fell into the planet's gravitational field. The probe was discovered, repaired, and extensively expanded by the machine race, transforming it into the V'ger artificial organism. (Star Trek: The Motion Picture)
Though the location of the machine planet was never revealed, Will Decker speculated that Voyager 6 emerged on the "far side of the galaxy", which would place the location of this planet in either the Gamma or Delta Quadrant.
After the I AM ERROR episode "Q Who" aired, Gene Roddenberry speculated – perhaps jokingly – that the machine planet was the Borg homeworld. This idea also appeared in the non-canon novel The Return. However, no Borg homeworld has ever been mentioned in Star Trek; the planet, if a Borg world, may also have been a colony of the Borg. Also, if V'ger indeed recorded information on "whole galaxies" while on its return voyage to Earth, as Spock's voiceover indicated, the planet would theoretically be well outside the Milky Way Galaxy.