There is a statement space is a vacuum. But it is not always true.Because space ahead on earth’s atmosphere has 8 particle/cm-3.So that statement is 100% not true.Not only that beyond the interplanetary area ,space between stars which scientist called it as Interstellar space also have some certain density .If there is certain density what it contains?.That should be talk further.
The medium which located in between stars it called as inter stellar medium .This medium contains two main components.They are gas and dust.Interstellar gas contains 99% of ISM while dust contains 1%. But this 1% do huge work.
This dust is different than people see in day to day life. It consists of clumps of atoms and molecules composed with most of time in carbon, silicon.As well as sometimes they are made up with some ‘dirty ice’,a frozen mixture of ordinary water ice contaminated with trace amount of ammonia,methane,and other chemical compounds. These are made like grains and those grain structure has graphite like structure with carbon mixed with silicon and metals (like MgSiO3-olivine).These dust are made from out of interstellar gas, because of its composition materials are underabundant in gas. Following figure get more idea about dust grain.
Most of time these are sorted in elonged or rodlike , perhaps needle shaped .That shows by following figure.
But most valuable thing is diameter of grain takes 500 nm or 1000 A0 .Because wavelength of blue colour and higher energy waves get small wavelength than that.
With this diameter higher frequency waves blocked and that create significant incidents. When a light beam which has all wavelength comes toward this grain it transparent only for comparable or larger wavelengths only.These grains are opaque for below that.Density of this grains are normally 1 million /m-3.For that most of high energy UV,X-rays, gamma rays, optical rays(near blue region) are blocked and lower energy Radio,IR,near red region optical rays are comes through it.So if there is a star behind interstellar cloud most of high energy will loss up and that incident called as intersteller extinction.That blocked light most of time scattered by this dust .So cloud will see in blue region ,because near blue region light scattered in optical range by dust mostly.Then cloud called as reflection nebula and source light stars will see dim or at red region stars.This called as interstellar redding.It can easily figure in following picture.
Sometimes these elonged shape dust particles create polarized light.Because there is a term like scatter polarization.See below.