Matter and energy are the two basic components of the universe, and 95% are currently unobservable. The outer space is almost a perfect vacuum full of cosmic voids.
What is a Vacuum?
A vacuum is defined as a space devoid of all matter. This word comes from the Latin word "vacuus," which means "vacant" or "void". Vacuums are nothing, or the lack of anything. They are cold, empty spaces that makeup almost all of the universe's space. Any region with fewer particles than air at atmospheric pressure is considered a vacuum.
The outer space has very low density and pressure and is the closest physical approximation of a perfect vacuum. However, no vacuum is truly excellent. A perfect vacuum is impossible because, according to quantum theory, energy fluctuations called 'virtual particles' constantly pop in and out of existence, even in 'empty' space.
Even the most remote region of outer space contains dust, gas, radiation, gravity, and many particles. In the Solar System, on average, space contains five atoms per cubic centimeter. There is about one atom per cubic centimeter in interstellar space between stars. Meanwhile, intergalactic space between galaxies is 100 times less.
If we try to suck all the particles out of a specific volume, it still cannot get empty. There are things such as dark matter, gravity, and vacuum fluctuations which cannot be sucked out. Still, it can be said that outer space is very close to empty compared to the atmosphere of the Earth. So what makes the outer space nearly empty?
Why is Space a Vacuum?
Regardless of size, every particle of matter exerts a gravitational attraction on all other matter particles. For small particles of matter such as hydrogen atoms and large distances on the astronomical scale, the force of gravity is weak but not zero. With enough time, gravity will condense giant clouds of gas in space even if the gravitational force is so weak.
Soon after the Big Bang, the universe was filled with an almost uniform soup of hydrogen and helium. Over billions of years, most of these gases were pulled by gravity into stars. Hydrogen and helium atoms fused inside the stars to form the heavier elements up to iron. The more giant stars died in an explosive supernova, creating all the naturally occurring ingredients heavier than iron.
These heavier elements were spewed into space. Over time, they were condensed down under gravity to form small clouds and rocks. Meanwhile, the clouds and rocks attracted each other gravitationally to form moons, asteroids, and planets. In other words, outer space is nearly empty because most matter has been transformed into an asteroid, moon, Earth, or star under the influence of gravity.
The expansion of the universe does not cause the vacuum of outer space. The universe's expansion only affects the distance between galaxies but does not increase the distances between objects inside a galaxy. The atoms in the human body, the creatures on Earth, the planets in our solar system, and the star systems in a galaxy are all far too strongly bound by gravity and electromagnetism to be affected by the universe's expansion.
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