No one can tell you for sure what the appendix does. No one knows how deep into the Earth life goes. No one knows how high into the sky life goes. No one is sure what the mites that live on human foreheads do, though they are there while you are reading.
Most species on Earth remain unnamed, not to mention totally unstudied. New species are easy to find in Manhattan, walking around alongside celebrities. No one can tell me what the species of bacteria living on my body, hundreds of species, are doing. No one can say for sure if there is another, yet to be discovered, domain of life. Parasites in my body might be affecting my behavior, and even the sorts of things I write late at night.