Comparing Gravity and Big Bang Theory

[Blade_Runner]

I don't know much about big bang theory except that it says that the universe is expanding..
What i don't understand is that why isn't the gravitational force pulling everything together ?
Is there any other force which is leading to the expansion??

 

[mathman]

Once the expansion got going, the theory was that gravity would slow it down. However about fifteen years ago it was discovered that the expansion is speeding up. Since then scientists have been looking for an explanation. The current theory is that there exists something called dark energy (which makes up about 75% of the universe) which is causing the acceleration of the expansion. It is supposed to be related to Einstein's cosmological constant. This whole idea is still somewhat tentative!