I have a doubt on the expansion of the universe. In the recent years, it has been observed that the universe is expanding in an increasing speed. So the existence of dark energy was suggested. Actually is the speculation that the universe is expanding in an increasing speed because of dark matter necessary of true? When we throw a ball upwards, first its velocity increases from zero to a certain level and then it decelerates into zero and again increases and finally comes to zero. Then couldn't the current accelerating expansion be considered as the initial increase in velocity as in the case of the ball? Why is it believed that dark energy drives the expansion. I would like to know there reason behind it. If this is far fetched and such questions aren't allowed in PF then please forgive me.