  After 25 years of the prediction of the possibility of observations, and
despite the many hundreds of well studied transiting exoplanet systems, we are
still waiting for the announcement of the first confirmed exomoon. We follow
the ``cascade'' structure of the Drake equation, but apply it to the chain of
events leading to a successful detection of an exomoon. The scope of this paper
is to reveal the structure of the problem, rather than to give a quantitative
solution. We identify three important steps that can lead us to discovery. The
steps are the formation, the orbital dynamics and long-term stability, and the
observability of a given exomoon in a given system. This way, the question will
be closely related to questions of star formation, planet formation, 5 possible
pathways of moon formation; long-term dynamics of evolved planet systems
involving stellar and planetary rotation and internal structure; and the proper
evaluation of the observed data, taking the correlated noise of stellar and
instrumental origin and the sampling function also into account. We highlight
how a successful exomoon observation and the interpretations of the expected
further measurements prove to be among the most complex and most
interdisciplinary questions in astrophysics.
