A time-varying cosmological "constant" A is consistent with Einstein's equations provided matter and/or radiation is created or destroyed to compensate for it. Supposing an empty primordial Universe endowed with a very large cosmological term, matter will emerge gradually as A decays. Provided only radiation or ultrarelativistic matter is initially created, the Universe starts in a nearly de Sitter phase, which evolves towards a FRW regime as expansion proceeds. If, at some cosmological time, the cosmological term begins increasing again, as presently observed, the expansion will accelerate, and matter and/or radiation will be transformed back into dark energy. It is shown that such accelerated expansion is a route towards a new kind of gravitational singular state, characterized by an empty, conformally transitive spacetime in which all energy is dark.
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