It’s almost entirely made of gas…
99% of the material in interstellar space is gas (the other 1% is dust), with roughly 90% of the atoms in the gas being hydrogen, 9% helium and 1% heavier elements.
…but it’s otherwise mostly empty
Interstellar space has an average density of one atom per cubic centimetre, although there are regions with higher densities, like nebulas, that have millions of atoms per cubic centimetre.
It’s the birthplace of stars
The regions of interstellar space where clouds of cold molecular and atomic hydrogen are present can form stars through gravitational attraction over hundreds of millions of years.
It contains intensely dark regions
Clouds of dust known as dark nebulas can form in interstellar space, which absorb light in a process called interstellar extinction and appear darker than other regions of space.
It also has beautiful reflection nebulas
Some regions of dust and gas actually reflect light and create fantastic nebulas like the Witch Head Nebula (pictured), which appears blue due to the scattering caused by its dust particles.
Image credit: NASA/STScI Digitized Sky Survey/Noel Carboni