  The concept of absorption of a photon moving backwards in time is reexamined,
and it is found that its interpretation as absorption is wrong, its original
interpretation rather as an emission being restored. The result is that the
anticosmos is invisible rather than indistinguishable from the cosmos, and as
such it is qualitatevely a candidate for constituting the dark matter.
Newton&#180;s law of gravitation is then generalized to refer either to matter
(of the cosmos) and/or antimatter (of the anticosmos), and quantitatevely is
found for the anticosmos to constitute the dark matter, as being (absolutely)
equal in ammount of antimatter to the ammount of matter in the cosmos.Finally,
a mechanism of gravitational repulsion from antimatter is proposed to explain
the observed deceleration of Pioneer 10 and Pioneer 11, and an analytical way
is exposed to calculate the position, as well as its (anti-)mass, of the
responsible anti-body.
