Our Universe is Expanding

Scientists know that our universe is expanding, But how fast?

From the start of big bang our universe is expanding. So it is logical to assume that the expansion will stop someday. Universe has its own momentum and energy by which it is expanding. But someday it may stop and start to collapse due to the gravitational pull of the stars and galaxies.

Scientists are assuming that there is some force that is preventing our universe from collapsing into itself. In order to know that force we have to know the material our universe is made of. Some of us may think we have already known about the materials such as hydrogen, oxygen, electron, proton, neutron etc. But in reality we know very little of the composition of our universe.

Scientists know that our universe is expanding. But how fast? The question hasn't been answered yet. Without knowing this answer it is difficult to predict the fate of our universe. But why? What is the problem of finding the rate of some expansion?