Interstellar gas is being used up to create stars faster than it is being replenished by the venting of engorged stars and supernova. The reason for this is that while all of the gas that goes into initially forming a star is lost to the interstellar pool, only some of it is returned, because much of its matter winds up locked up in collapsed stars: white dwarfs, neutron stars, and black holes. Hence, eventually the gas is going to run out, and eventually all the stars are going to die and not be replaced. Owing to decaying orbits, sometime after that happens they are almost all going to be eaten by the black hole that lies at the center of every galaxy.
So someday, all that will be left is a big dead universe with a bunch of blackholes in it.
(Source: The Inexplicable Universe: Unsolved Mysteries | Neil deGrasse Tyson )
String theorists think that blackholes might be creating new universes, but at anyrate, that is a pretty bleak result for our universe.
One potential solution, one endorsed by world renowned physicist Michio Akaku, is to harness the power of dark energy.
Dark energy is tearing the universe apart. It is a ubiquitious force that is slowly inflating all of space at a rate ofIt is a constant per given volume of space, it continuously causes the universes's volume to expand, hence it is continually becoming more and more powerful. HubbleSite - NewsCenter - Refined Hubble Constant Narrows Possible Explanations for Dark Energy (05/07/2009) - The Full Story74.2 kilometers per second per megaparsec (error margin of ± 3.6).
It is extremely diffuse, but it is, as far as we know, infinite. Meaning, it defies the conservation of energy. Hence, if there were an energy efficient way to collect it, it would grant you unlimited energy that you could use to do whatever you want. Since energy can be converted into matter, it would provide whatever civilization harvested it the ability to expand and survive forever.
Now, we don't really understand it very well. We don't know what is causing it. All we know are its effects. But if the model aforementioned is true, is it possible to use dark energy to save the universe? Or is it impossible to efficiently collect something that diffuse?