  I argue that Einstein overlooked an important aspect of the relativity of
time in never quite realizing his quest to embody Mach's principle in his
theory of gravity. As a step towards that goal, I broaden the Strong
Equivalence Principle to a new principle of physics, the Cosmological
Equivalence Principle, to account for the role of the evolving average regional
density of the universe in the synchronisation of clocks and the relative
calibration of inertial frames. In a universe dominated by voids of the size
observed in large-scale structure surveys, the density contrasts of expanding
regions are strong enough that a relative deceleration of the background
between voids and the environment of galaxies, typically of order 10^{-10}
m/s^2, must be accounted for. As a result one finds a universe whose present
age varies by billions of years according to the position of the observer: a
timescape. This model universe is observationally viable: it passes three
critical independent tests, and makes additional predictions. Dark energy is
revealed as a mis-identification of gravitational energy gradients and the
resulting variance in clock rates. Understanding the biggest mystery in
cosmology therefore involves a paradigm shift, but in an unexpected direction:
the conceptual understanding of time and energy in Einstein's own theory is
incomplete.
