The space between the stars in our Milky Way is not completely empty, but is crossed by huge clouds of gas and dust.
The density of the clouds is very low, depending on the region and composition, it varies between a few atoms per m3 and 100,000 molecules per cm3. For comparison, 1 cm3 of air at sea level contains the unimaginably large number of 2.6 x 1019 molecules.
In visible light, the clouds are divided into several groups: Emission Nebula (ionized gas leads to glowing), Reflection Nebulae (reflecting the light of the surroundings) or Molecular Clouds (absorbing the light of stars and nebulae behind them). Planetary Nebula (excited to glow) and Supernova Remnants (glow when gases cool down) are formed at the end of life of stars.