Short answers, pulled from the story.
Outer space is the expanse beyond Earth's atmosphere and between celestial bodies, a near-perfect vacuum of predominantly hydrogen and helium plasma. It is permeated by electromagnetic radiation, cosmic rays, neutrinos, magnetic fields and dust, and most of its mass-energy is an unknown form called dark matter and dark energy.
The baseline temperature of outer space, set by the background radiation from the Big Bang, is 2.7255 K. Gas temperatures vary widely, from 1 K in the Boomerang Nebula to between 1,200,000 and 2,600,000 K in the solar corona.
Outer space has no definite starting altitude, but the Kármán line at 100 km above sea level is used as the conventional boundary in space treaties and aerospace records. It is named after Theodore von Kármán and marks where a vehicle can no longer generate enough aerodynamic lift to support itself.
The Outer Space Treaty bars any claims of national sovereignty and calls outer space the province of all mankind, free for all states to explore. The United Nations General Assembly passed it in 1963, and it was signed in 1967 by the USSR, the USA and the UK, with 105 state parties by 2017.
The lack of pressure is the most immediate danger, and above the Armstrong line at about 19.14 km, exposed fluids such as saliva, tears and the liquids in the lungs boil away. Long-term weightlessness causes muscle atrophy, bone loss, a slowing cardiovascular system and a weakened immune system, while cosmic ray radiation raises the risk of cancer and organ damage.
Yuri Gagarin of the Soviet Union first achieved crewed Earth orbit in 1961 aboard Vostok 1. The first humans to escape low Earth orbit were Frank Borman, Jim Lovell and William Anders in 1968 aboard Apollo 8, which reached a maximum distance of 377,349 km from Earth.