  We use two simple independent gedankan experiments to show that the
holographic principle can be understood intuitively as having its origin in the
quantum fluctuations of spacetime. Applied to cosmology, this consideration
leads to a dynamical cosmological constant of the observed magnitude, a result
that can also be obtained for the present and recent cosmic eras by using
unimodular gravity and causal set theory. Next we generalize the concept of
gravitational thermodynamics to a spacetime with positive cosmological constant
(like ours) to reveal the natural emergence, in galactic dynamics, of a
critical acceleration parameter related to the cosmological constant. We are
then led to construct a phenomenological model of dark matter which we call
"modified dark matter" (MDM) in which the dark matter density profile depends
on both the cosmological constant and ordinary matter. We provide observational
tests of MDM by fitting the rotation curves to a sample of 30 local spiral
galaxies with a single free parameter and by showing that the dynamical and
observed masses agree in a sample of 93 galactic clusters. We also give a brief
discussion of the possibility that quanta of both dark energy and dark matter
are non-local, obeying quantum Boltzmann statistics (also called infinite
statistics) as described by a curious average of the bosonic and fermionic
algebras. If such a scenario is correct, we can expect some novel particle
phenomenology involving dark matter interactions. This may explain why so far
no dark matter detection experiments have been able to claim convincingly to
have detected dark matter.
