Curses!
Oh, how sad, all this time I was looking forward to colliding with the Andromeda galaxy in 4.5 billion years!
Funny, a couple of decades ago the consensus was the exact opposite, that we were headed towards the Big Crunch. Of course that was before "dark matter" was taken seriously. The problem with Quantum Mechanics is that unlike most of the physical sciences, very little of it is empirical, they make most of it up as they go along.
The expansion of the universe becomes noticeable only on a scale way larger than the one of a galaxy (at least one hundred million light-years, one thousand times the size of the Milky Way). It has very little or no influence on local dynamics. In fact the collision between our galaxy and Andromeda will happen in a few billions of years (three billion or something approaching, the Sun will still exist at that time), but it has nothing to do with the expansion of the universe.
Indeed, I was well aware that the two events are entirely unrelated. I was just referring to the lively dynamics of The Local Group to illustrate the remoteness of the timescale considered when we talk about The Universe expanding indefinitely. Yes the Sun will likely still be kicking when we crash into Andromeda, but by then - at least according to most models-it will have expanded to the size where it will have engulfed the Earth- or pushed the Earth's orbit farther out into the solar system- in either case The Earth's atmosphere will have long been stripped away by the Solar wind. As a cooler red giant It will have fused virtually all of its hydrogen and most of its remaining supplies of helium and will be puttering out as it struggles to make do with carbon and oxygen.
Dark matter (which contributes to a potential Big Crunch) is unrelated to Dark Energy (which contributes/is the accelerating expansion of the universe). Generally the evidence for either is empirical; we're still figuring out the math to find the rest of their influence and how they work... we're still working on the theories (i.e. theoretical evidence).
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 . . . .