"If the gravitational attraction of all the matter within the observable horizon were high enough, the expansion of the universe would slow, and then reverse eventually. The universe would then contract, with about the same duration as the expansion. Eventually, all matter and energy would be compressed back into a gravitational singularity.
However, recent experimental evidence (namely the observation of distant supernovae as standard candles, and the well-resolved mapping of the cosmic microwave background) has—to considerable surprise—shown that the expansion of the universe is not being slowed down by gravity, but instead, accelerating, suggesting that the universe will not end with a Big Crunch, but will instead expand forever. Although some scientists have contested this theory,[1] most cosmologists have considered the evidence of an accelerating universe to be conclusive since 2002."
I was going to ask for what the catalyst would be, but seeing as it's unlikely, I guess it won't matter.