In high school biology, we are taught that there are three types of life: eukaryotes (that's us, and most everything else we often think of as life), bacteria, and archaea (extremophiles and other very primitive life forms). But some scientists are pretty sure that there are entirely different, undiscovered lifeforms that could be prevalent on Earth, and they remain undescribed because we're not good at looking for them.
In a new paper published in Science, Tanja Woyke and Edward Rubin of the Department of Energy's Joint Genome Institute note that "there are reasons to believe that current approaches [to discovering life] may indeed miss taxa, particularly if they are very different from those that have so far been characterized."
In other words, there may be life out there that doesn't even use the four DNA and RNA bases that we're used to; there may be life out there that has evolved completely separately from everything that we have ever known to exist; there may be life that lives in places we haven't even looked.
maybe undiscovered life is using a genetic code we don't understand
Discovering a new type of bacteria or virus or, hell, even a mammal at this point isn't exactly news. It happens all the time, and there are tens of millions of species out there that aren't described. Scientists can at least place newly discovered life into specific categories, but that's because they've been using the same methods of finding it for decades.
For the rest of the story: http://motherboard.vice.com/read/why-scientists-think-completely-unclassifiable-and-undiscovered-life-forms-exist