Dark energy is perhaps the most mysterious thing in the cosmos, and yet it accounts for 73% of the observable universe. What's more, the amount of dark energy seems to be increasing...and that could ultimately rip apart the entire universe.
For the purposes of this post, we're going to be treating dark energy in much the same way astrophysicists do - as the current best explanation for certain features of the universe, albeit one that is still not well understood. For background on just what dark energy is (as much as anyone can say that) and why science needs it, I'd direct you to Dr. Dave Goldberg's excellent primer.
The question we'll be looking at is what dark energy might mean for the future of the universe. One thing we do know about it is that it appears to have a constant density throughout the universe over time. Since the universe is expanding, that means the amount of dark energy has to in some way increase to keep up with it - in fact, since so much of the expanding universe is empty space, it means that the proportion of stuff (that's a technical term) in the universe that is dark energy will increase over time - and it's already a pretty huge percentage of that.
The simple explanation for that is that dark energy is somehow a feature of space-time itself, so it expands in kind as the universe expands. That means that the "energy" part of dark energy is a misnomer, but that's the least of our worries. Assuming this is true, we can then say that dark energy is associated with negative pressure, meaning that it acts as a repulsive force driving the expansion of the universe.
In this conception, dark energy is basically just "the cost of having space", meaning that any given volume of space requires a certain amount of dark energy in order to, well, exist. (I know this is vague. There's still a lot we don't know.) As far as cosmologists can tell, the density of dark energy and the pressure of dark energy are in a 1:1 ratio - or, since the pressure of dark energy is negative, a 1:-1 ratio. This is referred to as the equation of state for dark energy, and so the commonly accepted value is -1.
So here's the question: will that -1 stay the same for the rest of the universe's existence? Right now, we have no idea, but we do have a sense of what would happen if it did change. If the density of dark energy decreased over time, that would slow down the rate of expansion - if the ratio dropped made it to -1/3, then the expansion would cease entirely. That would potentially lead to the Big Crunch, in which we see a reversal of the Big Bang. That was once the most popular scenario among astrophysicists for the end of the universe, but it's only possible if the dark energy density decreases.
But what if the density of dark energy increased? As it stands, the universe will just keep on expanding forever. Is there a more extreme fate than that? Oh, you'd better believe it. If the ratio dropped still further to -2 or -3, the dark energy density would approach infinity within a finite amount of time.
So what does that mean? Well, remember that the observable universe is just whatever in the larger universe has had enough time for its light to reach Earth, and the expansion of the universe means that some parts of the universe have moved forever beyond our gaze. Now imagine everything is pulled so far apart by the cosmic expansion that each individual bubble of observable universe is no larger than a galaxy, or the solar system, or our planet, or the room around you...or even down to the individual atoms.
That is what infinity density means, and it would cause what's known as the Big Rip, in which all structures in the universe down to the subatomic particles get torn apart. If the ratio dropped to even just -1.5, the universe would suffer this fate within the next 22 billion years.
That's not a cheery thought, but a miniature version of this process could end up doing the universe a world of good. If the ratio of pressure and density of dark energy can be variable throughout the universe, then it would be possible for the cosmos to experience Little Rips, in which only certain parts of the universe get torn apart. This would actually work to "reboot" the entropy of the universe, as all the newly split apart material can begin again the process of forming stars and galaxies.