Outer space is not empty; it is filled with a thin soup of gas, dust, and radiation.

The material between stars is called the interstellar medium. It is made of 99% gas and 1% dust. Most of this gas is hydrogen and helium. For comparison, the air on Earth is much thicker. One cubic centimeter of air contains 25 quintillion molecules.NASA probes like Voyager 1 and Voyager 2 have measured the density of space. They used special tools to listen to the vibration of electrons in the gas. These vibrations sound like a low hum. This proves that space is not a total void.Space also contains light called the Cosmic Microwave Background. This light has been traveling since the Big Bang. Scientists Arno Penzias and Robert Wilson discovered it in 1964. There is also dark matter, which makes up a large part of the universe.Even if you removed every atom, space would still have energy. Tiny particles pop in and out of existence constantly. This is called a quantum fluctuation. These particles make the vacuum of space a very active place.The thin gas in space still has physical effects. It creates a small amount of drag on satellites orbiting Earth. It also provides the raw material needed to form new stars. Space is a near-vacuum rather than a perfect one.