Why is space a vacuum?

The universe is an almost absolute vacuum . Why is that? Let's learn about the vacuum of the universe together !

You may have heard that space is a vacuum and wondered, “So why doesn’t it suck us away from Earth, like a vacuum cleaner sucks dirt off a carpet?” The answer goes back to how the universe began and how Earth formed.

Definition of vacuum

Before we get into that, let's talk about what a vacuum is and isn't. A vacuum isn't actually an object—it's the absence of anything. So when someone says "space is a vacuum," what they really mean is that it's almost empty. As a simple example, if you took a jar on a spacewalk and sealed it, there probably wouldn't be a single atom inside it.

Robot vacuum cleaners also maintain a vacuum, but it is different from the vacuum of space: Dust and hair are drawn into the vacuum due to the pressure difference caused by suction.

A cramped start

The universe wasn’t always empty. For the first 400,000 years after the big bang, every inch of the universe was filled with a dense, swirling mixture—mostly hydrogen. All these tightly packed atoms were constantly bouncing off each other, making the initial mixture extremely hot. But over time, space expanded, making the mixture thinner (as would happen if you poured real mixture onto a table). The thinner mixture also cooled and began to solidify.

Gravity makes it difficult for matter to stay scattered; eventually, everything will clump together. The first clumps of matter formed purely by chance—some patches of space happened to contain more matter than others. And since all matter has gravity, the more matter there is in a patch, the more it pulls everything else toward it. Over the eons, these regions of matter gathered more and more matter. Eventually, some clumps reached their maximum mass, and their pull became so strong that they rapidly collapsed into stars. The debris they left behind then formed the orbiting planets. The regions between them were left with a near-absolute vacuum of space.

Gravity creates the “environment” we live in, but it is also what prevents things on Earth from flying off into space. The vacuum of space actually exerts a pull on the air in our atmosphere. But just as you can pull your finger out of a vacuum tube with a little force, gravity can prevent the air in our atmosphere from being sucked out into space.

Cabin pressure

When you're watching a science fiction movie and there's a hole in the side of a spaceship, things start to get sucked out. That's because the pressure inside the ship is higher than the very low pressure of space, so there's nothing holding it back to counteract the vacuum. All matter gravitates, but a ship doesn't have enough matter to pull air in—it would take something as big as the Earth.

The universe isn’t actually empty. If you threw out a big bag of trash instead of a small bottle, you might catch a few hydrogen atoms that have escaped the gravitational pull of all the stars and planets. And like the incredibly hot, dense gas of the early universe, the hydrogen that remains in space is so loosely compressed that it’s freezing cold.

The vacuum of space is also why it is silent. Sound is just vibration, and without any air molecules to vibrate, sound cannot travel.

Next time someone mentions the "cold vacuum of space," you can explain why it's such an unappealing place.