Why Doesn't Star Trek Ever Leave The Milky Way Galaxy?

Key Points:

• "Star Trek" stories predominantly take place within the Milky Way galaxy, divided into four quadrants, with Earth located in the Alpha Quadrant near the Beta Quadrant border. This setting allows most known alien species to originate from within the same galaxy, maintaining narrative cohesion.
• A fictional "galactic barrier" surrounding the Milky Way serves as a plot device preventing starships from traveling to other galaxies, as it damages ships attempting to cross and blocks communications, thereby limiting exploration beyond the galaxy.
• Although some episodes depict brief ventures outside the Milky Way, such as reaching the Triangulum Galaxy (M33), intergalactic travel remains impractical within "Star Trek" due to vast distances and warp engine limitations, making such journeys take thousands to millions of years.
• The franchise’s focus on humanoid species is explained by the in-universe concept of ancient Progenitors seeding the Milky Way with their DNA, facilitating storytelling and production by using human actors and reducing the need for costly special effects for non-humanoid aliens.
• Ultimately, "Star Trek" remains set in the Milky Way for narrative clarity, production practicality, and to explore human themes effectively, as the galaxy itself offers ample scope for adventure without venturing into the vast, mostly empty intergalactic medium.