A vacuum is the absence of matter. The pressure within a vacuum is 0 pascals. Longitudinal waves cannot travel through a vacuum, but transverse waves can.

Space is often used as an example of a vacuum, but it is not a perfect one. Lets take the example of the earth's atmosphere. The highest level of the atmosphere is known as the Exosphere. Here molecules are constantly exiting (and re-entering) the atmosphere. This is the same for planets (with an atmosphere) across the universe.

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