  It has been argued that a Universe governed by Eddington-Born-Infeld gravity
can be compatible with current cosmological constraints. The extra fields
introduced in this theory can behave both as dark matter and dark energy,
unifying the dark sector in one coherent framework. We show the various roles
the extra fields can play in the expansion of the Universe and study the
evolution of linear perturbations in the various regimes. We find that, as a
unified theory of the dark sector, Eddington-Born-Infeld gravity will lead to
excessive fluctuations in the Cosmic Microwave Background on large scales. In
the presence of a cosmological constant, however, the extra fields can behave
as a form of non-particulate dark matter and can lead to a cosmology which is
entirely compatible with current observations of large scale structure. We
discuss the interpretation of this form of dark matter and how it can differ
from standard, particulate dark matter.
