So interstellar molecules are only found in the dark centers of dense nebula of gas and dust. Just as electrons orbit around the nucleus in quantized energy states, atoms can rotate around each other, also in quantized speeds. Changes in the rotation, by collisions or interactions, will emit photons. Radiation from interstellar molecules is a good tracer of the dense, thick regions of the interstellar medium. These dense regions are usually the sites of protostar formation and collapse of large clouds of gas to form into star clusters.
Interstellar dust is produced in the envelopes around red supergiant stars.
Stellar winds and the planetary nebula phase eject this dust into the interstellar medium. Dust particles are mostly carbon and silicate grains that are a few microns in size. Although small, they completely absorb any light that strikes them and are heated by collisions with gas molecules. They will be warmed to the ambient temperature of the surrounding region and re-radiate that energy in the far infrared. Although solid objects, dust grains are not immune to destruction.