The Expanse is set in a future in which humanity has colonized much of the Solar System, but does not have the means to travel to other systems. In the asteroid belt and beyond, tensions are rising between Earth's United Nations, Mars, and the outer planets.
The series initially takes place in our solar system, using many real world locations such as Ceres and Eros in the asteroid belt, several moons of Jupiter, with Ganymede and Europa the most developed and small science bases as far out as Phoebe around Saturn and Titania around Uranus. There are also well established, domed settlements on Mars and the Moon.
Central to the series is the existence of The Protomolecule, an advanced alien terraforming tool that arrived in our solar system billions of years ago aboard a Bracewall probe. Much like the Exsurgent Virus, it’s capable of infecting and transforming biological life forms it encounters, using them to build strange and inhuman structures.