Xenoarchaeology is a hypothetical form of archaeology that exists mainly in science fiction works concerned with the physical remains of past (but not necessarily extinct) alien life and cultures. It is not currently practiced by mainstream archaeologists, due to the obvious lack of examples.
Read more about Xenoarchaeology: Etymology, Justification, History, Planetary SETI, Probe SETI, or SETA, Dysonian SETI, Fringe Theories, Science Fiction