What is a Dark Nebula?
A dark nebula is a dense interstellar cloud of molecular gas and dust particles that block all visible light from background stars and various cosmic objects, most notably in bright emission nebulae. Dark nebulae are extremely cold as with some such as 'Barnard 68', a dark nebula in the constellation Ophiuchus, is at an estimated -257 degrees celsius. Because of the molecular composition and sheer density of these clouds, star formation is common.