Inter Stellar Medium is the ‘stuff’ that exists between solar systems.
It is the ‘fabric’ of Space.
Science has believed for years that Space is empty; that it is a void and a vacuum.
Science used to define the fabric of Space as the ‘ether’.
In the absence of any physical evidence of the ether, this theory was dropped during the 20th century.
The ether became the province of Spirituality and Spiritualism, when Physics became a purely physical science.
With the advent of unmanned space probes reaching the outer reaches of our Solar System, it now appears that Space is not empty.
The extent of our Sun’s cosmic influence is now defined as the Heliosphere.
Apparently the Heliosphere is no longer empty; it is now full of ‘solar wind’.
Also, Outer Space is no longer empty either because the Inter Stellar Medium of Outer Space meeting the Inner Stellar Medium of the Heliosphere is now eminently measurable.
Nothing is allowed to become scientific until it is physically measurable.
Spiritual Mediums know differently.