| The answer involves temperature. Cold things do not emit visible light. Just turn off the light and see how little light is emitted by your favorite wall. As you warm things up, they begin to emit light in relatively long wavelengths. Infrared light has a longer wavelength than visible light and is given off by hot objects (think of a glowing hot electric burner on your stove). Increasingly hot things radiate energy in increasingly shorter wavelengths, from visible light (filament in a light bulb) all the way up to x-rays, depending on temperature.
In space, stars are extremely hot and give off lots of visible light and ultraviolet (UV) light. Planets are cold. We only see them when sunlight reflects off their surfaces. This reflected light is much dimmer than the original light source, so the only time we see an illuminated planet (or moon) is when it is reasonably close or if we use powerful telescopes.
Most gas and dust clouds are cold and therefore do not emit light. In order to see a distant cloud, we need to either have brilliant light reflecting off of the cloud's surface or have the cloud backlit by starlight such that it forms a shadow in an otherwise bright area. Many galactic clouds are visible due to excellent backlighting, but clouds associated with star nurseries can show both reflected light and light emitted by parts of the clouds heated by intense radiation streaming from nearby stars.
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