Nu-World was the master plan of Ted Castle and a group of other rich and powerful people on Earth to create a new Earth because the current Earth would be uninhabitable in a mere thirty years. It is an exact replica of the Earth, down to the tiniest detail. Through massive funding and the technological expertise of Alyssa Moy Castle, Nu-World was to be the ultimate paradise without disease, poverty or weapons.
Nu-World was to be protected by an artificially designed creation of Alyssa known as Conserve and Protect (CAP), but the robot went rogue and it took Reed Richards and his Anti-Galactus Suit to destroy it. Ted and Alyssa asked Richards to join their team of scientists, but Reed chose to try to solve the current Earth's problems rather than abandon it.
Alyssa eventually learned that her husband had only planned for the rich and famous to be transported to Nu-World in order to repopulate the race. She went to the Invisible Woman and told her of Ted's true intentions.
When the New Defenders came to the current time, they planned on transporting the remaining six billion people of their dying future Earth back to the past. Mr. Fantastic and the Invisible Woman reprogrammed their time machine (the Galactus Engine) to send all the survivors to Nu-World, thus giving them a nirvana to live in and ruining Ted Castle's plans.
Shortly after, the Sun of Nu-World collapsed on itself forming a black hole. This accelerated time greatly, eight years passing in two weeks normal time. The bleak hope of the future caused the inhabitants to wage war until Lightwave and Alex Ultron took control of the planet, using Natalie X to control as many of the residents as they could. To top it off, the portal to leave Nu-World was broken and no help could be reached.
Ted Castle was able to obtain a power source from Valeria Richards that he could use to pilot Nu-World away from the black hole.
When the present Galactus learned of his demise in the future, he went to Nu-World and destroyed it.