Interstellar matter is all uncoalesced matter in the regions between stars. It is comprised of solid dust, neutral gas and ionized plasma.
The interstellar medium contains ionised elements (predominantly carbon, silicon and sulphur), neutral elements (most of which is hydrogen, helium, nitrogen, oxygen, neon and argon) and condensed dust grains (primarily aluminium, calcium magnesium, and iron). The lightest elements, hydrogen, helium and lithium, were formed during the Big Bang. Heavier elements were created within stars by nucleosynthesis before being ejected into interstellar space by stellar winds and supernova explosions.
The interstellar matter closer to our solar system includes matter created after the original nebula from which our sun was formed and is expected to be richer in heavier elements and neutron rich isotopes. 
Interstellar dust obscures most nearby regions in visual and ultraviolet wavelengths, reradiating the absorbed energy in the far-infrared (FIR). This provides about 30% of the total luminosity of the Milky Way Galaxy. Interstellar dust FIR radiation removes the gravitational energy of collapsing clouds making it possible for stars to form.
Interstellar dust reduces ultraviolet (UV) radiation which causes molecular dissociations. Removing the influence of UV provides for the formation of H2, the most abundant interstellar molecule. Dust also controls the temperature of the interstellar medium (ISM) by cooling. Conversely it also provides heating through electrons that are photoelectrically ejected from other dust grains.
Galaxies contains significant amounts of tenuous matter, unevenly distributed throughout interstellar space. It is composed of gas (atoms, molecules, ions, and electrons) and dust (tiny solid particles). Interstellar matter obscures, reddens and polarises of starlight by forming absorption lines in stellar spectra, and through other emission mechanisms (both over a continuum and at specific wavelengths). The effect of interstellar matter on star light was first realised by Giovanni Cassini, an astronomer in the 17th century who recognised dust in interplanetary space, and around the sun, through telescopic observations.
Interstellar matter comprises a small percentage of the total mass of a galaxy, about 10 − 15 % of the total mass of the galactic disk of the Milky Way, and tends to concentrate near the galactic plane and along the spiral arms in spiral galaxies.
Approximately 50% of interstellar matter forms discrete clouds which occupy about 1 − 2 % of the interstellar volume. 
- There are three types of interstellar clouds:
- Dark clouds - Essentially very cold molecular gas ranging in temperature from 10 − 20 K and block off the light from background stars;
- Diffuse clouds - Cold atomic gas at about 100 K, almost transparent to the background starlight. However, at a number of specific wavelengths they give rise to absorption lines;
- Translucent clouds - Composed of molecular and atomic gases and have intermediate visual extinctions.
Between the clouds there are three different forms of interstellar matter:
- warm (mostly neutral) atomic matter about 104 K,
- warm ionized matter about 104 K
- hot ionized matter about 106 K 
- Interstellar matter is very thinly distributed, tenuous. In the region of the Sun it averages about 2.7 x 10−24 g cm−3, ranging from about 1.5 x 1026 g cm-3 in the hot medium from approximately 2 x 10−20 to 2 x 10−18 g cm−3 in the densest molecular regions. This is about the same as a single hydrogen atom per cubic centimetre or nearly 20 orders of magnitude below that of the Earth’s lower atmosphere. 
- The chemical composition of interstellar matter, by mass, is about 70.4% hydrogen, 28.1% helium and 1.5% heavier elements, similar to the ratios measured in the Sun, other disk stars and meteorites.
- Gaseous phase interstellar matter, as determined by spectrographic analysis, apparently lacks heavier elements in the same ratios. One explanation for this is that the heavier elements are bound in solid dust grains of interstellar matter. Dust comprises about 0.5 to 1% of the mass interstellar matter.
Neutral atomic gas
Warm ionised gas
Hot ionised gas
Outer cloud dust
Inner cloud dust
- Sampling interstellar matter Jet Propulsion Labs, NASA
- Massive star formation Churchwell, E. Harvard-Smithsonian Center for Astrophysics
- Evolution of Pre-main sequence stars Palla, F. Harvard-Smithsonian Center for Astrophysics
- Stardust Spacecraft Reaches for Cosmic Dust (2002) Jet Propulsion Lab, NASA
- Interstellar dust and extinction Mathis, John (1990) Annu. Rev. Astron. Astrophys. 28: 37-70
- SPACECRAFT - Cassini Orbiter Instruments - CDA Jet Propulsion Lab, NASA
- The Interstellar Environment of our Galaxy Ferriére, Katia M. (2001) Reviews of Modern Physics 73 (4): 1031-1066.
- Visual extinction, sometimes referred to as attenuation, in astronomy refers to the reduction of the intensity of radiation as a consequence of absorption and scattering, in other words, the dimming of starlight as it passes through the interstellar medium, Another way of putting this is that visual extinction is the loss of light from an object as a consequence of absorption or scattering by an intervening medium. An example is the atmospheric extinction of light from stars near the horizon.
- Interstellar Medium University of Tennessee