  The huge amounts of undetected and exotic dark matter and dark energy needed
to make general relativity work on large scales argue that we should
investigate modifications of gravity. The only stable, metric-based and
invariant alternative to general relativity is $f(R)$ models. These models can
explain primordial inflation, but they cannot dispense with either dark matter
or dark energy. I advocate nonlocal modifications of gravity, not as new
fundamental theories but rather as the gravitational vacuum polarization
engendered by infrared quanta produced during primordial inflation. I also
discuss some of the many objections which have been raised to this idea.
