One of the biggest questions in science is "What will happen to the Universe"?
The fate of the Universe is a balance between these two. If the expansion wins, everything will expand forever. If gravity wins, the expansion will stop, and the Universe will collapse. Astronomers are trying to work out how much mass there is in the Universe to help to answer this question - the more mass, the more likely it is that gravity will win.
The best results at the moment make us think that the Universe is just on the balance point and will never quite collapse!