Space is an almost perfect vacuum, nearly void of matter and with extremely low pressure. In space, sound doesn’t carry because there aren’t molecules close enough together to transmit sound between them. Not quite empty, bits of gas, dust and other matter floats around «emptier» areas of the universe, while more crowded regions can host planets, stars and galaxies.

From our bound perspective, outer space is most often thought to begin about 62 miles (100 kilometers) above sea level at what is known as the  This is an imaginary boundary at an altitude where there is no appreciable air to breathe or scatter light. Passing this altitude, blue starts to give way to black because oxygen molecules are not in enough abundance to make the sky blue.