  The Kerr-Schild (KS) geometry is linked tightly with the auxiliary
\emph{flat} Minkowski background. Nevertheless, it describes many curved
space-times and the related physical models, starting from cosmology and black
holes to the microworld of the spinning elementary particles and the
pre-quantum structure of vacuum fluctuations. We consider here a KS model of
the Bubble Universe -- a semi-closed Universe with a rotating de Sitter (or
anti-de Sitter) space embedded in an external flat space-time. When the
solution has two horizons, it may also be interpreted as an Universe inside a
black hole. In micro-world the KS geometry yields a model of the spinning
particle consistent with gravity and describes a pre-quantum twistorial
structure of space-time with the beam-like fluctuations of metric consistent
with the beamlike fluctuations of electromagnetic vacuum. These light-like
twistor-beam excitations are consistent with gravity and generalize the known
pp-wave solutions. Following Wheeler's estimations of the density of vacuum
fluctuations we arrive at the general conclusion that Universe should be flat
and have a zero cosmological constant. It enforces us to return to an
`effective flat geometry' filled by the electromagnetic background radiation.
