"There are in fact a hundred billion other galaxies. Each of which contains something like a hundred billion stars."

Carl Sagan was quite accurate in this learned approximation when he uttered it in the early 1980s. The galactic and stellar populations of the Universe are staggering! But if the eloquent astrophysicst were alive today, he would be eager to share an equally fascinating development: that there may -- in fact -- be countless galaxies essentially devoid of stars!

Dark galaxies, as they are fittingly called, are still in the realm of hypothesis, but observational evidence is increasingly weaving them into reality. Composed of only hydrogen gas, dust, and dark matter, and without visible stars, they are nearly impossible to detect. The best evidence for their existence arrived just three years ago. Astronomers aligned with the European Southern Observatory spotted a quasar -- an unimaginably massive and radiant stellar object formed by galaxy collisions -- illuminating twelve large collections of hydrogen gas, causing the gas in each to faintly fluoresce. The astronomers interpreted those gas collections as dark galaxies.

Without the quasar's natural flashlight, the dark galaxies would otherwise have remained unseen. Zeroing in on those galaxies, what the team of astronomers did see, were low-mass regions of space roughly 16,500 to 19,800 light years across (roughly one-fifth the size of the Milky Way) each with amounts of gas (mostly hydrogen) about one billion times more massive than our Sun.

As dark galaxies reside in our roiling universe, they are not destined to remain hidden, or even around at all. In fact, they may serve as fuel for more conventional, star-studded galaxies.

"In our current theory of galaxy formation, we believe that big galaxies form from the merger of smaller galaxies. Dark galaxies bring to big galaxies a lot of gas, which then accelerates star formation in the bigger galaxies," Sebastiano Cantalupo, a Research Team Leader at the Institute for Astronomy, ETH Zurich, told the Kavli Foundation.

Martin Haehnelt, a Professor of Cosmology and Astrophysics at the University of Cambridge, concurs.

"We expect the precursor to the Milky Way was a smaller bright galaxy that merged with dark galaxies nearby. They all came together to form our Milky Way that we see today."

Considering that dark galaxies have been, for the most part, observed billions of light years away, and thus billions of light years back in time, there is some question as to their prevalence today. It's entirely possible that they formed only in the wake of the Big Bang, and since then have been swallowed up by larger galaxies.

But a dark galaxy could also be next door. In 2009, Matthew Nichols and Joss Bland-Hawthorn of the University of Sydney, Australia suggested that Smith's Cloud (pictured below), a cloud of hydrogen gas 9,800 light years long and 3,300 light years wide located 40,000 light years away from Earth just outside of the Milky Way, may be a dark galaxy. Regardless, in approximately 27 million years it will merge with the Milky Way, lending its bounty of stellar fuel to our galactic home.

(Images: Shutterstock, ESO/M. Kornmesser, Bill Saxton/NRAO/AUI/NSF)