Ok. You all remember the dude at the end of the Avengers. You know, the guy with the purple skin and the chin? The same guy who shows up briefly in The [Guardians of the Galaxy](movie:424073). I've heard lots of people asking the questions,"Who is Thanos?" or,"I thought he was the biggest baddie of them all. Why hasn't he done anything yet?" I'm going to answer those questions for those of you who are wondering.
"Who is Thanos?"
Thanos is the last of his species called the Titans. He is referred to as The Mad Titan. He gained this title because he is the one who destroyed his race.
Why did he do that? you may ask. I'll tell you. Ever since he was young, Thanos was obsessed with death. This obsession later lead to another obsession. He became obsessed with Death. Not death as in, when you die, but as in the person. Thanos falls in love with the woman Death, who is the essence of death.
Thanos will do pretty much whatever it takes to impress her including destroying his own species. On that same note, we'll move onto our next question...
What is Thanos waiting for?
Yes, Thanos is the biggest villian in the MARVEL universe. He just hasn't gotten his full power yet. Let me explain. Being obsessed with impressing Death, he vows to her that he'll destroy all life in the universe just for her. Although being really powerful, being able to withstand just about anything and fire cosmic energy galore out of his hands, he just isn't powerful Enough to destroy the universe. He needs the Infinity Gauntlet.
The Infinity Gauntlet is THE most powerful weapon in MARVEL. It is seen in Thor, in the trophy room. The Gauntlet is powered by six stones(or Gems) called the Infinity Stones. The stones that have appeared in the Cinematic Universe are The Tesseract, The Aether, and The Orb.
It is seen, in the movies, that Thanos is using various different people to collect these Stones for him. It hasn't been going so well, but believe me he's about to get all of 'em. It's gonna be messy.
Thanos hasn't been showing himself to our heroes yet because he can't fight them all just yet. Just wait for a little while longer and you'll see him in action! :)
Hope this helped some people out a bit. Let me know in the comments.