The galaxy, the first of its kind to be detected, has been named VIRGOHI21. It could only be found using radio telescopes because there are no stars to give light, the team behind the discovery said.
A dark galaxy is an area containing a large amount of mass that rotates like a galaxy, but contains no stars.
Similar objects previously discovered have turned out to contain stars when studied with high-powered optical telescopes, while others have been found to be the remnants of two galaxies colliding.
But when the team of scientists from the UK, France, Italy and Australia who found VIRGOHI21 studied it, they uncovered no trace of any stars and no nearby galaxies which would suggest a collision.
Professor Mike Disney, a member of the Cardiff University-led team, said: "As Sherlock Holmes said ‘When you have eliminated the impossible, whatever is left - however improbable - must be the truth’."