Just as most of the matter in the universe is thought to be “dark matter,” much of Earth is populated by a kind of microbial analogue: microorganisms that are known to exist but have never been grown in a laboratory.

A new study, published last September in mSystems, suggests such microbes could account for up to 81 percent of all bacterial genera that live outside the human body. These little-known organisms could hold the secrets to new tools for treating disease and could help us understand life in extreme environments, such as those on other planets.