Curses!
Theoretical evidence, precisely- which begs the question,: Aren't the terms "Dark Matter", as well as "Dark Energy" for that matter merely theoretical metaphors for the multitude of observations which seem to contradict our current mathematical explanations of gravity, and our understanding of energy? Rather than embodying a literal description of the phenomena observed, these terms seem to be convenient substitutes for the gaps in our empirical knowledge.
Yes. I am pretty sure are metaphors. Dark energy seems to be another name for the accelerated expansion itself, and doesn't address an underlying cause for this observed acceleration.
Since we don't know what 'Dark Energy' is it is still possible that it isn't a constant force. Theoretically it could decrease in energy over time eventually losing its ability to accelerate the expansion of the universe against gravity. In other words, gravity could eventually get the upper hand to slow and then reverse the expansion (if dark energy is losing force over time) . . .
All to say we can't say CONCLUSIVELY that the Universe will continue expanding forever . . . .