Particularly dense molecular clouds, like LDN 483, qualify as dark nebulae because of this obscuring property. (sciencedaily.com)
Other scenarios involve gravitational perturbations, either caused by passing stars, a solar system entering and exiting our galaxy's gravitationally dense spiral arms, or even via interactions with dense molecular clouds. (sciencemag.org)
Stars begin their life in dense molecular clouds. (unlv.edu)