Space is the near-limitless expanse that exists beyond Earth’s atmosphere, beginning roughly 100 kilometers (62 miles) above sea level. It is not truly empty. While far more barren than any environment on Earth, space contains thin wisps of gas, dust, radiation, and energy fields that stretch between stars and galaxies. Understanding what space actually is means looking at where it starts, what fills it, and how it behaves.
Where Earth’s Atmosphere Ends and Space Begins
The internationally recognized boundary of space sits at 100 kilometers altitude, a line known as the Kármán line. The World Air Sports Federation adopted this standard after extensive calculations showed that around this altitude, the atmosphere becomes too thin for conventional aircraft to generate lift. There’s no visible wall or sharp cutoff. Instead, the air gradually thins across several layers until the molecules are so sparse they barely interact with each other.
Earth’s uppermost atmospheric layer, the exosphere, stretches from about 700 to 10,000 kilometers above the surface. At these altitudes, molecules are so scarce that the air no longer behaves like a gas at all. Particles drift freely and occasionally escape into space entirely. So while the Kármán line is a useful legal and engineering boundary, the true transition from atmosphere to space is a long, gradual fade rather than a clean break.
What Fills the “Emptiness”
Space is often called a vacuum, and compared to the air you breathe, it essentially is one. At sea level, a cubic meter of air contains around 25 trillion trillion molecules. In the space between stars, that same volume might hold just a few hundred thousand atoms, mostly hydrogen and helium. These two elements dominate the universe’s chemical makeup: hydrogen accounts for roughly 73% of all normal matter by mass, helium about 25%, and everything else, all the oxygen, carbon, iron, silicon, and dozens of other elements, makes up barely 2%.
Between galaxies, space gets even emptier. Cosmic voids, the enormous gaps between the web-like filaments of galaxy clusters, can stretch tens to hundreds of millions of light-years across. Their density drops to as low as 10% of the universe’s average, making them the closest thing to true nothingness that exists. Yet even these regions contain trace amounts of gas and are permeated by radiation and magnetic fields.
The Baseline Temperature
Space has a background temperature of about 2.7 Kelvin, which translates to roughly minus 270 degrees Celsius (or minus 455 degrees Fahrenheit). This faint warmth is the leftover glow from the Big Bang, a sea of microwave radiation that fills the entire universe. It’s the coldest natural temperature that empty space reaches, though specific regions near stars or other energy sources can be dramatically hotter, and shadowed surfaces in deep space can cool even further through radiation.
How Space and Time Work Together
Space isn’t just a passive container that objects move through. In modern physics, space and time are woven together into a single framework called spacetime. This fabric is flexible. Massive objects like stars and planets curve the spacetime around them, and that curvature is what we experience as gravity. Objects moving through curved spacetime follow the straightest possible paths available to them, which happen to be curved paths. That’s why the Moon orbits Earth and why light bends around massive galaxies.
This idea, central to Einstein’s general theory of relativity, replaced the older notion that gravity was simply a force pulling objects toward each other across empty space. Instead, matter shapes the geometry of space itself. The more mass concentrated in a region, the more spacetime curves there. Near a black hole, the curvature becomes so extreme that nothing, not even light, can escape.
What We Can’t See
All the hydrogen, helium, stars, planets, and dust in the universe, everything made of atoms, accounts for only about 5% of the total energy content of the cosmos. The remaining 95% is invisible. Dark matter, which makes up about 27%, exerts gravitational pull on galaxies and holds them together but doesn’t emit or absorb light. Dark energy, comprising roughly 68%, appears to drive the accelerating expansion of the universe. Neither has been directly detected or fully explained, which means the vast majority of what space “contains” remains one of the biggest open questions in science.
What Space Does to the Human Body
Unprotected exposure to the vacuum of space is survivable only for a very brief window. The most immediate danger is oxygen deprivation: without pressure to keep air in your lungs, you lose consciousness in less than 15 seconds. If you were holding your breath at the moment of exposure, trapped air would expand rapidly and tear the delicate tissue inside your lungs, making the situation far worse.
Within seconds, water in your soft tissues begins to vaporize under the near-zero pressure, causing significant swelling throughout the body. Your skin is tough enough to keep you from bursting apart, and your eyes won’t explode, but moisture on exposed surfaces like your tongue and airways boils and cools rapidly. Dissolved gas in your blood forms bubbles that block circulation, and after roughly one minute, blood flow effectively stops.
Animal experiments and at least one well-documented accident confirm that recovery is possible if pressure is restored quickly. In 1965, a technician at NASA’s Johnson Space Center accidentally lost suit pressure inside a vacuum chamber. He blacked out after about 14 seconds. His suit was repressurized within 27 seconds, and he regained consciousness almost immediately. His only lasting effect was a loss of taste sensation that took four days to return. Beyond a couple of minutes of exposure, though, the damage becomes irreversible.