The words “deep space” conjure up images of exploring far corners of the galaxy. This romantic idea is somewhat correct; deep space refers to space beyond our Solar System. Deep space can sometimes refer to interstellar space, which is any space outside a star and its planetary system. Interplanetary space is the space in a planetary system up to the heliopause where the interplanetary space gives way to interstellar space. The heliopause is part of the heliosphere, which is a kind of shield protecting the Solar System from radiation. Deep space is a combination of interstellar space and interplanetary space from all other solar systems except our own.
Interstellar space, and deep space for that matter, is not the empty vacuum that fictional portrayals have led us to believe. It is filled with interstellar medium (ISM). Interstellar medium is gas and dust that occupies interstellar space. It is a very thin mixture of cosmic rays, magnetic fields, ions, grains of dust, and other molecules. The density of the material changes depending on where it is. It is denser closer to the planetary system with an average density of a million particles per each cubic meter. The gas in the interstellar medium is composed of approximately 89% hydrogen, 9% helium, and 2% of other heavier materials including tiny amounts of metals.
Astronomers have been trying to determine the nature of interstellar space for centuries – at least since the 1600’s – but their efforts were hampered by the limited tools and technology they had available. The interstellar medium is important to astrophysicists because it helps them determine how quickly a solar system uses up its gases, and from that, how long its lifespan of active star formation is.
In addition to interstellar space, deep space includes intergalactic space. Intergalactic space refers to the space between galaxies. Intergalactic space is almost completely empty and very close to a total vacuum. The density of the material in intergalactic space – intergalactic medium (IGM) – differs in different spots. There is a higher density of intergalactic medium closer to star systems because much of the medium comes from solar winds and other debris from the planetary system. Astronomers believe that the gas in the intergalactic medium is ionized gas as a result of its relatively high temperatures. Deep space holds a certain allure hinting at the unknown and mysterious, which is one reason why it has always fascinated people.
Astronomy Cast has an article on space junk.