  There is a strong possibility that the particles making up the dark matter in
the Universe have a mass below 1 eV and in many important situations exhibit a
wave-like behavior. Amongst the candidates the axion stands out as particularly
well motivated but other possibilities such as axion-like particles, light
scalars and light vectors, should be seriously investigated with both
experiments and theory. Discovery of any of these dark matter particles would
be revolutionary. The wave-like nature opens special opportunities to gain
precise information on the particle properties a well as astrophysical
information on dark matter shortly after a first detection. To achieve these
goals requires continued strong support for the next generations of axion
experiments to probe significant axion parameter space this decade and to
realize the vision of a definitive axion search program in the next 20 years.
This needs to be complemented by strong and flexible support for a broad range
of smaller experiments, sensitive to the full variety of wave-like dark matter
candidates. These have their own discovery potential but can also be the test
bed for future larger scale searches. Strong technological support not only
allows for the optimal realization of the current and near future experiments
but new technologies such as quantum measurement and control can also provide
the next evolutionary jump enabling a broader and deeper sensitivity. Finally,
a theory effort ranging from fundamental model building over investigating
phenomenological constraints to the conception of new experimental techniques
is a cornerstone of the current rapid developments in the search for wave-like
dark matter and should be strengthened to have a solid foundation for the
future.
