  Most dark matter (DM) models set the DM relic density by some interaction
with Standard Model particles. Such models generally assume the existence of
Standard Model particles early on, with the DM relic density a later
consequence of those interactions. Perhaps a more compelling assumption is that
DM is not part of the Standard Model sector and a population of DM too is
generated at the end of inflation. This democratic assumption does not
necessarily provide a natural value for the DM relic density, and superficially
leads to too much entropy in the dark sector. We address the latter issue by
the late decay of heavy particles produced at early times, associating the DM
relic density with the lifetime of a long-lived state. We ask what it would
take for this scenario to be compatible with observations in what we call
Flooded Dark Matter (FDM) and discuss several interesting consequences. One is
that DM can be very light and furthermore, light DM is in some sense the most
natural scenario in FDM as it is compatible with larger couplings of the
decaying particle. Moreover, the decay of the field with the smallest coupling
and hence the longest lifetime dominates the entropy and possibly the matter
content of the Universe, a principle we refer to as 'Maximum Baroqueness'. We
also show that the dark sector should be colder than the ordinary sector,
relaxing the free-streaming constraints on light DM. We will discuss the
implications for the core-cusp problem in a follow-up paper. FDM also has
interesting baryogenesis implications. One possibility is that both DM and
baryon asymmetries are simultaneously diluted by a late entropy dump.
Alternatively, FDM is compatible with an elegant non-thermal leptogenesis
implementation in which decays of a heavy RH neutrino lead to late time
reheating of the Standard Model and provide suitable conditions for creation of
a lepton asymmetry.
