Space is often referred to as a vacuum because it is an extremely low-density environment with a lack of air or any other matter that we typically encounter on Earth. There are a few reasons why there is no air in space:
Gravity: On Earth, the force of gravity holds our atmosphere in place. The gravitational attraction between Earth and the molecules in the atmosphere keeps them from drifting away into space. However, in space, where the gravitational forces are significantly weaker or absent, there is no force to confine the gases that make up the Earth's atmosphere.
Escape velocity: For a gas molecule to escape from a planet's atmosphere, it needs to reach a certain speed known as the escape velocity. The escape velocity of Earth is about 40,270 km/h (25,020 mph). Gas molecules in the Earth's atmosphere generally have lower velocities and are held in place by gravity. However, some molecules in the upper atmosphere, particularly lighter ones like hydrogen and helium, can reach escape velocity and escape into space over time.
Intergalactic medium: Space is not entirely empty but contains extremely low-density particles, including atoms, ions, and cosmic dust. However, their concentration is so sparse that they are not considered air or a breathable atmosphere. These particles are part of the intergalactic medium, which fills the gaps between galaxies.
It's important to note that while space is a vacuum, it is not a perfect vacuum. There are still some particles present, but their density is incredibly low compared to what we consider air on Earth.