  The significance of past-pointing four-vectors and negative energies in
general relativity is discussed. The sign of the energy is not absolute, but
relative to the four-velocity of the observer, and every particle/observer
always measures its own mass as positive. It is shown that the description of
the interaction of past-pointing and future-pointing matter in general
relativity requires two metric tensors for self-consistency. This aspect of
general relativity might account for the observations that led to the proposal
of "dark energy" and non-baryonic "dark matter".
