The Kree: Beginnings
The Kree and Skrulls have been at war a very long time. About 25,000 years ago, the Kree arrived in Sol's system and discovered that the dominant species on Earth had their DNA tampered with by the Celestials. They began their own experiments on Earth's inhabitants adding some of their own DNA to that of the Earthlings to grow something more: soldiers and weapons to use in their war against the Skrulls. The experiments that were successful produced Inhumans, Human-Kree hybrids that, when exposed to Terrigen (a limited resource that is crystalline when inert and gaseous when activated) can gain powerful, sometimes terrible, potential and abilities. Yet there's no guarantee. Some die. Some few unfortunates come out unchanged. The very unfortunate are left hairless and devolve back to a state of early mankind: strong but of reduced intellect. These are the Alpha Primitives, viewed by their society as near-mindless Inhumans good for nothing more than labor. After adolescence a member of Inhuman society, if selected, steps into the Chrysalis Chamber and are ceremoniously exposed to the Terrigen Mists. None can predict the outcome beforehand except under one circumstance: those who are not Inhuman, those without the hybridized DNA, die.
Tibet: Life on Earth
For thousands of years, the Inhumans watched from Attilan, their hidden city in the mountains of Tibet. Life flourished among the ancient birthplace of their people. The Royal family worked tirelessly for the survival of their people while the Genetic Council shaped their evolution and destiny. While the rest of the world waged war against each other, the Inhumans worked in secret to keep points of interest around the globe safe from harm. It was a beautiful utopia. But it couldn't last.
The relationship between China and Russia has always been strained and the tension had been rising rapidly since the development of the Sentinels. Both countries had heavily invested in them with an eye not only to control their own people but to use against enemies as well. With both pollution and the threat of war against powered persons starting to occur around the globe, Attilan's welfare was becoming endangered. There had already been sorties between Russia and China uncomfortably close to the city, including artillery bombardment, some of which caused damage. The humans didn't need to know Attilan was there in order to be a serious threat.
Maximus, brother of the current king, had been biding his time and chose this situation to act. He commanded an army of Alpha Primitives, those regressed Inhumans who were kept deep in the catacombs below ground as a source of free labor and not a little shame, and succeeded in overthrowing his brother, Black Bolt. His reign was short lived though as an explosion rocked the city, shattering from the inside the great defensive shield built years ago by the Mad Prince himself, releasing Terrigen into the atmosphere and causing the creation of Nuhumans all around the world. In the chaos that followed, Black Bolt and those loyal to him took back the throne. With the threats of human war, exposure of their existence, pollution, and now the shortage of Terrigen, the king made a very hard decision: evacuate not just the city, their ancient homeland, but the Earth as well.
Dark Side of the Moon: A New Beginning
A very long time ago, the Skrulls created the Blue Area on the dark side of Earth's moon as a testing ground for the Kree and Cotati. When the Kree returned to the solar system, they used it to perform their experiments on the humans. The successful Inhumans were housed there for a time and knowledge of it passed down through the millenia. As their situation on Earth worsened in the last century, Attilan's rulers had those few who could teleport transport builders, shapers and laborers to it and construct shelters. Shelters became an outpost and an outpost became a city, albeit a small one. Black Bolt made the decision: 2015 was the year the Inhumans would leave their ancestral home.
Smaller than their former home in the Himalayas, the news city was filled to near capacity. The caste system was more crucial than ever to keep harmony as they worked on expanding it to try to accommodate their numbers. Quarters, while elegant, were cramped but they were safe in an environment that would not contaminate and kill them for now. They used their abilities and technology to keep track of what was happening on the Earth and Black Bolt weighed what direction to take his people in and how to help the Nuhumans who only exist because of their actions. Out of a sense of responsibility, some Inhumans have been sent to Earth to aid and teach their new cousins.
Like humans on Earth, the Inhumans rebuild their lives and from the Moon, keep an eye out for extraterrestrial threats, namely the Kree. Quietly, overtures to the United Nations were made and their existence disclosed to a small number under very controlled conditions. Some are working closely with SHIELD, specifically SWORD, to assist with what knowledge they possess and what technology they are willing to share, in order to safeguard their planet. The humans might not know yet that the Kree are the enemy but the Inhumans do. While Attilan's location and secrets are their own, Earth is still their home. They have moved from its surface but they have not abandoned it.