Interstellar space is the space between star systems in a galaxy, and it is not empty. It contains a variety of matter and radiation, including gas in ionic, atomic, and molecular form, dust, cosmic rays, neutrinos, charged particles, atoms, molecules, dark matter, and photons. The matter in interstellar space is extremely dilute, with an average density of about 1 atom per cubic centimeter. The interstellar medium (ISM) is the collective term for everything that is in interstellar space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field.

The beginning of interstellar space is defined as the place where the Suns constant flow of material and magnetic field stop affecting its surroundings, which is called the heliopause. The heliopause marks the end of the region created by our Sun called the heliosphere, which is created by the solar wind that sends a constant flow of particles and a magnetic field out into space at over 670,000 miles per hour. Once a spacecraft has traveled past the inner edge of the heliosphere, it enters the termination shock, which is like white-water rapids in a stream, and then the heliopause, an area where the internal pressure of the Suns effect balances with the outside pressure of the interstellar medium.

In summary, interstellar space is the space between star systems in a galaxy, and it contains a variety of matter and radiation. The beginning of interstellar space is defined as the place where the Suns constant flow of material and magnetic field stop affecting its surroundings, which is called the heliopause.