This reality is designated as Earth-5001. This universe bears many similarities to Earth-616, the main timeline of the Marvel Comics universe, although there are branches of divergence.

Recent History of Earth
Following the first appearance of costumed superheroes in the mid-20th century, there was a Golden Age of superheroes that lasted several decades. That era came to an end when the Scarlet Witch, a founding member of the Avengers, became afflicted by madness. Her betrayal of the Avengers caused that team's disbandment, and soon afterward, she disappeared.

Shortly after that, a breakout at the Raft maximum-security prison led to the escape of dozens of supervillains. The ensuing battles between heroes and villains brought great collateral damage to the public, sparking the Superhuman Registration movement. The opposition from some heroes to this movement led to a superhuman Civil War between two opposing sides.  Leading the pro-Registration movement was Iron Man (Tony Stark), and opposing him was Captain America (Steve Rogers), who led the anti-Registration forces. The Pro-Registration heroes won in the end, but realizing the error of his ways, Stark helped pass the 50 State Initiative, a compromise piece of legislation that allowed individual states to choose how they would enforce superhero activity.

But the superhero community remained bitterly divided in the wake of the Civil War, and it was only a matter of time before evil forces turned circumstances to their advantage. All over the globe, alien Skrull impostors revealed themselves in positions of power, using the chaos and confusion of their sudden appearance to steal ancient artifacts of immense power called Infinity Stones. Their plan came to fruition when their leader, the Mad Titan Thanos, defeated Earth's united superhumans and used the stones to wipe out half of all life in the universe. He then disappeared, leaving his Skrulls to rule over the chaos-stricken Earth and only a small number of superheroes left to oppose them.

This brings us to the present. Unfortunately, even in these troubled times, NYC mayor Norman Osborn has drawn a hard line against vigilantes, and is doing his best to stamp out the remaining heroes operating in his city. He stands in direct opposition to these masked do-gooders and is doing his very best to steer public opinion against them. Never ones to surrender, the remaining superheroes in NYC have formed a new team: the Secret Avengers. With Osborn in power, the superhero community in disarray, and super-powered threats like Carnage suddenly on the loose, their heroism is now being put to the ultimate test...