Roughly 98% of interstellar space is permeated by diffuse gas that is not dense enough to form interstellar gas clouds. This intercloud gas is divided into two categories based on temperature: the warm intercloud medium with a temperature of around 8,000 Kelvin, and hot coronal gas so named because its temperature is comparable to the million Kelvin gas found in the Sun’s corona.
Astronomers believe that intercloud gas is heated to such extreme temperatures when a massive star ends its life in a supernova explosion. When the star explodes it creates shock waves which heat the gas and carve out cavities in the surrounding interstellar medium. For this reason, in the galactic disk at least, hot coronal gas tends to exist in pockets of very low density, with less than 0.01 atoms/cm³.

It appears that our own Solar System is located inside one of these bubbles of hot coronal gas. The so-called Local Bubble is estimated to be about 650 light years across, have a density of ~0.0065 atoms/cm³, and to have formed from the explosion of a supernova 100,000 years ago.
Hot coronal gas can only be observed from space-based observatories since both the X-ray emission given off by the hot gas, and the signature interstellar absorption lines of O VI (five times ionised oxygen) and N V (four times ionised nitrogen) are blocked by the Earth’s atmosphere.