The other side of the coin to the Big Crunch would be the Big Rip (although, as always with physics, there are more than two sides to the coin. It is a multi-dimensional quantum coin, but more on that later). One of the major discoveries that derailed the the Big Crunch theory was the observation that the expansion of the universe is actually accelerating. Physicists discovered this by accident during a supernova survey, causing them to eventually come up with the concept of "dark energy". The theory is that there is some kind of undetectable force or energy in the universe that counteracts the effects of gravity (generated by all the "dark matter" that we similarly have no idea about). We don't actually know what dark energy, it's basically an ominous, catch-all term for "we have literally no idea" at the moment - but that doesn't mean it won't have consequences. If the expansion of the universe is indeed speeding up, this dark energy will eventually cause it to tear itself limb from limb. First, the large structures, such as galaxies, will fly apart. Then everything in them will begin to break down, the stars, planets and little apes living on those planets will be ripped apart into their constituent atoms, which will in turn be ripped apart into pure energy which will dissipate and stretch in the infinite vacuum of nothingness. So that's cheery. Like the Big Crunch, there are a lot of uncertainties with this theory, so lets see if there are any more creative ways to destroy the universe that take your fancy.
