Thanos had been searching for the Infinity Stones for quite a while in the Marvel Cinematic Universe before acquiring all of them in Avengers: Infinity War. Marvel Studios had been teasing the existence of the Mad Titan as early as 2011's Thor. He appeared in a string of films since then, hinting that he was after all six elemental crystals, but it wasn't until Infinity War that it became clear what his plans were. 

Ever since physically appearing during The Avengers' post-credits scene, fans presumed that the Infinity Stones would factor in the franchise's long-term narrative plans. By Avengers: Age of Ultron, Marvel Studios officially laid the seeds for the culmination of the Infinity Saga with the Mad Titan deciding to look for the gems himself. Considering what's known about his arc in the MCU, he was already working on collecting the gems via surrogates such as Loki (Tom Hiddleston) and Ronan the Accuser (Lee Pace), and chances are that he was further motivated after Odin's (Anthony Hopkins) death. But his efforts in acquiring the crystals more likely preceded his first appearance in the universe. 

Related: Iron Man's Alternate Final Line Would've Ruined His Perfect Endgame Death

Despite being the primary villain of the Infinity Saga, much is still unknown about the Thanos (Josh Brolin). But based on what's known, after the destruction of Titan, the villain went on a quest to balance the universe in an effort to save it from the same fate as his home planet. When he got to Gamora's world, Zen-Whoberi, he was still executing his plans manually, leaving his minions to kill half of the planet's population. At this point, Gamora was around the age of 10, creating a roughly 20-year period between her abduction and Thanos sacrificing his so-called "daughter" on Vormir. By the events of 2012's The AvengersThanos already had the Mind Stone, so it's safe to say that he had been looking for the gems for around 15-20 years before he successfully acquired all of them in Avengers: Infinity War

Avengers Infinity War Gamora Walks With Thanos

Marvel Studios revealed limited information about Thanos and his Infinity Saga arc for a couple of reasons. Firstly, directors Joe and Anthony Russo were already tasked with bringing together the MCU in Avengers: Infinity War and were essentially forced to tackle only the most relevant aspects of the villain's story in the movie. While the 2018 blockbuster was both a critical and financial hit for the franchise, many thought that it would've been better if Marvel had given the Mad Titan his own origins projects instead of cramming his story into one of their most jampacked movies to date. It may have been intentional on the studio's part to prevent any more issues with MCU’s micro-continuity. While the universe has a clear overarching narrative, certain timeline details don't add up. So Kevin Feige and his team tend to not dwell with specific dates, especially as the franchise grows bigger. 

In any case, this is something that Marvel Studios can revisit moving forward. While the Marvel Cinematic Universe is already moving on from the Infinity Saga, the franchise can still revisit Thanos' arc including his quest for the Infinity Stones through a prequel or through scenes in future projects. The introduction of his brother, Eros (Harry Styles) in Eternals actually further motivates the studio to do just that. 

More: Phase 4 Shows The Avengers Learned Nothing From Thanos & Infinity War

Key Release Dates