Short answers, pulled from the story.
The interstellar medium consists of matter and radiation in the space between star systems within a galaxy. It includes gas in ionic, atomic, and molecular forms alongside dust and cosmic rays.
Voyager 1 became the first artificial object to reach the interstellar medium on the 25th of August 2012. Its twin Voyager 2 entered this region later on the 5th of November 2018.
Francis Bacon coined the word interstellar in the context of ancient theories regarding fixed stars. William Huggins used spectroscopy in eighteen sixty-four to determine nebulae consist of gas rather than solid bodies.
By mass ninety-nine percent of the interstellar medium consists of gas while one percent comprises dust grains. Hydrogen dominates the composition followed by helium with trace amounts of carbon oxygen and nitrogen.
Matter exists in multiple phases distinguished by ionization state temperature and density including cold dense clouds warm intercloud regions and very hot shock-heated gas. These phases reach temperatures where heating and cooling achieve stable equilibrium.