Well basically when all the matter was compacted in one single dot, it created such a astouding amount of energy that when it exploded the energy was released in a instant !! n because you know that energy must be conserved then the energy is used to create planets and Suns etc, n the rest of the energy is used as the motion of the planets which is parallel to the direction at which evrything was released into space so because everything was released, that meant that we will ocntinue to expand until we our momentum slows !!
We know why we are expanding because of the doppler effect!