The question of how galaxies form is a hot issue among astronomers. Computer models have helped them make educated guesses about how these groups of stars come to be, but it's been hard to validate these theories.

Now scientists have made a significant stride in confirming part of the story of galaxy formation. They believe they have found several examples of dark galaxies, dense clouds of gas that essentially don't have stars, so they can't be detected with optical telescopes.

"This may be considered the building blocks of galaxies," said Sebastiano Cantalupo, postdoctoral fellow at the University of California, Santa Cruz and lead author of the study. "What we believe is that this is an evolutionary phase into the whole history of galaxies."

Basically, a dark galaxy is the middle stage between a diffuse cloud of gas, and a galaxy as we know them, with a lot of stars.

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