  We propose that dark matter is not yet another new particle in nature, but
that it is a remnant of quantum gravitational effects on known fields. We
arrive at this possibility in an indirect and surprising manner: by considering
retarded, nonlocal, and Lorentzian evolution for quantum fields. This is
inspired by recent developments in causal set theory, where such an evolution
shows up as the continuum limit of scalar field propagation on a background
causal set. Concretely, we study the quantum theory of a massless scalar field
whose evolution is given not by the the d'Alembertian $\Box$, but by an
operator $\tilde \Box$ which is Lorentz invariant, reduces to $\Box$ at low
energies, and defines an explicitly retarded evolution: $(\tilde \Box \phi)(x)$
only depends on $\phi(y)$, with $y$ is in the causal past of $x$. This
modification results in the existence of a continuum of massive particles, in
addition to the usual massless ones, in the free theory. When interactions are
introduced, these massive or off-shell quanta can be produced by the scattering
of massless particles, but once produced, they no longer interact, which makes
them a natural candidate for dark matter.
