Outer space, commonly referred to as space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. It is not completely empty, but rather a near-perfect vacuum containing a low density of particles, predominantly hydrogen plasma, as well as electromagnetic radiation, magnetic fields, neutrinos, dust, and cosmic rays. The baseline temperature of outer space, as set by the background radiation from the Big Bang, is 2.7 kelvins (-270.45 °C; -454.81 °F) .

Outer space can be divided into different regions based on their proximity to Earth and the type of environment they contain. These regions include:

• Geospace: This is the region of outer space near Earth, including the upper region of the atmosphere and the magnetosphere. The Van Allen radiation belt lies within the geospace, and the space inside the magnetosphere is protected from radiation from the Sun.

• Interplanetary space: This is the space around the Sun and planets of the Solar System. It has the solar wind, a continuous stream of charged particles from the Sun, which creates a very thin atmosphere (the Heliosphere) for billions of miles or kilometers into space.

• Interstellar space: This is the physical space in a galaxy not occupied by stars or their planetary systems. It continues to the edges of the galaxy, where it fades into the intergalactic void. Most of the mass in this space is made up of single hydrogen atoms, fewer helium atoms, and a few heavier atoms formed in stars. Supernovae blow some of their atoms huge distances.

• Intergalactic space: This is the space between galaxies, which is mostly empty.

The modern concept of outer space is based on the "Big Bang" cosmology, which holds that the universe originated from a very dense form that has since undergone continuous expansion. The Outer Space Treaty provides the basic framework for international space law, covering the legal use of outer space by nation-states and including in its definition of outer space the Moon and other celestial bodies. The treaty states that outer space is free for all nation-states to explore and is not subject to claims of national sovereignty, calling outer space the "province of all mankind".