Background space, say inter stellar space that is within a given galaxy, the space between stars has about one molecule of H2 per cubic centimeter.
In SI units this is 106 H2 molecules per cubic meter. We call this the number density… one million molecules per cubic meter.
This may sound like a lot… but the number of molecules in one cubic meter of air at sea level (you are breathing that now) is about 3x 10 25 !!!!! That is 1020 times bigger!!!!
At any rate, photons from a laser that travel through vast columns of Interstellar Medium (ISM) will be scattered by this medium.
This scattering causes the the dimming and the reddening of light.
There are ways that space gas can change the color of light passing through it. I suspect that there is a region of space somewhere in the universe that has enough gas to do so with a laser, but there are some specific conditions and space has so little gas that the effect would be exceptionally weak on a per-distance basis.
Gas makes up about 99% of the mass of the collective "stuff" that exists in space between stars and galaxies called the interstellar medium . There isn't really a good number for how much mass this is for the entire universe, but the number density of gas atoms and molecules is generally on the order of 106 (i.e., 1 million) atoms/molecules per cubic centimeter of "empty space". As a comparison, air at Earth's surface has ~1019 atoms/molecules per cm3 (i.e., 10 trillion more).
The color of light is determined by the combination of wavelengths it contains. As an example, sunlight contains many different wavelengths of light. By splitting the light with a prism (and making a rainbow), one is able to see the different wavelengths in what is known as the spectrum. There are two ways that gas could change the color of light. One is by absorbing certain wavelengths of light, removing them from the traveling light. Another is by emitting light, thereby adding new wavelengths. The wavelengths that are absorbed or emitted are called the gas' absorption and emission spectra, respectively. Each gas species absorbs and emits only at specific wavelengths. As a result, the spectra can be used to identify which gases are present.
These are highly useful for astronomers trying to learn about material in a region of space and for identifying compositions of stars and atmosphere of planets and moons.
Light from a laser is different from sunlight in that laser light (usually) comprises only a single wavelength. If a laser is directed through gas, but the gas does not absorb that wavelength, then nothing will happen and the laser will pass through unchanged. On the other hand, if the gas absorbs that specific wavelength, then the laser's beam would diminish in intensity (get dimmer), potentially absorbing all of the light so that none of the laser beam passes through. Note though that whatever light is left would not change color. The absorption story does not end there either. After absorbing light from the laser beam, the electrons in the gas particles are in an excited, or high-energy, state. This is unstable and at some point the electrons will fall to lower energy levels. In transitioning down energy levels, light is emitted. This process of emission after absorption is called fluorescence.
Depending on the specific energy levels, the wavelength of the emitted light can vary. If the electrons falls in a single step from the excited state back to the initial state, then the emitted wavelength will be the same as what it absorbed from the laser, and no change in color will occur. BUT, if the electron falls in multiple steps, then the (multiple) emitted photons can have wavelengths different from that of the laser. There are special names for emission spectra of hydrogen depending on the wavelength of light emitted: the Balmer series for visible light, and the Lyman series for UV light.
Relating this back to the question of whether there is enough gas in space to change the color of a laser, I do not have a quantitative (or even definitive qualitative) answer. However, perhaps you have seen a laser pointer used on Earth. Over any distance, the beam does not apparently change color. Then remember that Earth's atmosphere is 1012 times more dense (as a particles/volume measure) than gas in space. To change the color of that laser, the gases that the laser is passing through would have to be matched to the wavelength of the laser light and the light would have to pass a very long distance through that gas to even potentially have an effect. And this assumes that the light is not first reflected or absorbed by other "stuff" in space, like dust.
Clouds of gas (really, plasma) do exist in space, but are extremely diffuse: most industrial-strength vacuums still have more gas in them than nebulae in space. However, even space outside of nebulae has gas in it - it's just tremendously diffuse. These clouds can, over great distances (light years) block light.
Changing the color of a laser is extremely difficult, and can't generally be accomplished by shining a laser beam through a material medium such as a gas. Typically, the medium will absorb the laser, rather than change its color.
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