The interstellar medium ISM is actually a name given to the gas and dust in interstellar space. It is a matter that actually exists between the stars within the galaxy and it is in the form of electromagnetic radiation. ISM consists of gas, plasma and dust. It has about 99% gas and 1% mass. ISM plays an extremely important role in astrological physics because of its highly important role between stellar and galactic spaces. This has been a topic of great importance since ages and is highly important. Before the theory of electromagnetism it was considered that an invisible luminiferous aether exists to carry light waves. These properties where discovered by Goldsmith and Habing in 1969. The extinction and reddening is all because of the type of medium. Thus ISM is a subject of great importance.