Interstellar Medium is matter that makes up the space between stars and star systems in a galaxy. Most people think space is a complete vacuum, making resources or matter completely absent or none at all. This is a common misconception, since space and the star systems really have matter.
Most mediums in outer space are gas and space dust that have varying densities. One fact is that 15% of the visible material in the Milky Way is made up of interstellar gas and dust. Most gases present in this type of space gas or matter are primarily made up of Hydrogen as its main element. It can either be in molecular or ionized forms, and is easy to use since it only has a volume of 1 atom per cubic meter. The gas form may be easy to handle, the thing is that the gas levels add up due to the vast and wide distances between star systems.
Interstellar mediums, mainly dust on the other hand, is not just average dust. It contains different substances and is extremely small or tiny. These particles are also irregularly shaped and are made of different elements, most likely iron or ice.
Picture this, Interstellar Medium