Biological dark matter is the term given to unclassified and poorly understood genetic material. Some of the genetic material may not fall under one of the three existing domains of life, suggesting a possible fourth domain of life that has not been discovered.

For example our stomachs contain viruses that keep the bacteria in check. According to one study only 19% of the viruses in our stomachs match anything in the genetic material databases. The other 81% is unknown due to the vast numbers of viruses, rapid evolution of viruses, and the inability to culture these viruses.

The NIH has the Human Microbiome Project that is trying to improve the understanding of microbial flora involved in human health and disease. They are trying to characterize the microbiome of heathy human subjects at five major body sites