Given the task definition and input, reply with output. In this task you will be given an answer to a question. You need to generate a question. The answer given should be a correct answer for the generated question.

In astronomy, the interstellar medium (ISM) is the matter and radiation that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field.

what do we mean by the interstellar medium