There are three ways we can see nebulae. Bright nebulae are visible due to either emissions from ionized gases, or from light reflecting off dust particles. In both cases, there must be stars within or very near the nebula to make it light up. There are also dark nebulae that we can see only as dark regions in front of the stars or bright nebulae behind them.
Emission nebulae do give off light, hence the name. There are also reflection nebulae, which reflect light of nearby stars. Many diffuse nebulae do both. A dark nebula does not give off light (or not very much), these can be detected because they block out light of whatever is "behind" them, like the "horse-head" part of the Horsehead Nebula, or the "Coalsack".
Some nebula are indeed dark - as black as coal dust, but most have stars within them or very close by and they shine by reflected light. Stars within nebula also excite atoms in the nebula causing electronic transitions that give off colored light - reds, greens, blues, etc
Answers & Comments
Verified answer
They reveal themselves through the background against stars and reflection brought about by very hot or very bright stars.
http://www.atlasoftheuniverse.com/nebulae.html
http://www.atlasoftheuniverse.com/darknebs.html
There are three ways we can see nebulae. Bright nebulae are visible due to either emissions from ionized gases, or from light reflecting off dust particles. In both cases, there must be stars within or very near the nebula to make it light up. There are also dark nebulae that we can see only as dark regions in front of the stars or bright nebulae behind them.
Emission nebulae do give off light, hence the name. There are also reflection nebulae, which reflect light of nearby stars. Many diffuse nebulae do both. A dark nebula does not give off light (or not very much), these can be detected because they block out light of whatever is "behind" them, like the "horse-head" part of the Horsehead Nebula, or the "Coalsack".
Some nebula are indeed dark - as black as coal dust, but most have stars within them or very close by and they shine by reflected light. Stars within nebula also excite atoms in the nebula causing electronic transitions that give off colored light - reds, greens, blues, etc
The nebula that we can see have already formed a star. The ultraviolet radiation ionizes the gas, turning it into plasma, and thus makes it visible.
Edit: Some nebulae may aslo reflect light from nearby stars.
The plasma is electrically charged and emits light. Think of one of those novelty plasma balls you touch to manipulate the electricity.
We can see that it blocks light.