https://phys.org/news/2018-05-dark-galaxies.html

To overcome the obstacle that their target objects are dark, the team used quasars as a flashlight of sorts. These emit intense ultraviolet light, which in turn induces fluorescent emission in hydrogen atoms known as the Lyman-alpha line. As a result, the signal from any dark galaxies in the vicinity of the quasar gets a boost, making them visible. Such fluorescent illumination has been used before in searches for dark galaxies, but Marino et al. searched the neighbourhood of quasars at greater distances than has been possible in earlier observations.

They acquired the full spectral information for each of the dark-galaxy candidates. Deep observations—10 hours for each of the six quasar fields they studied—enabled Marino and her colleagues to efficiently discern dark-galaxy candidates from other sources. From initially 200 Lyman-alpha emitters, a half-dozen regions remained that are unlikely to be normal star-forming stellar populations, making them robust candidates for dark galaxies.