Question: what causes gravity and why?

Sarah Cook answered on 19 Jun 2011:

I haven’t got a clue but I found this online which pretty much says no one else has a clue either?!?!

Isaac Newton first figured out the fundamental nature of gravity in the late 1600s. By unraveling the mysteries of planetary movement and Earth’s pull on its inhabitants, he described modern physics. But more than three centuries later, that’s still all we have: an understanding of the effect, with almost no grasp of the cause. Is gravity carried by an elementary particle? Is it some fundamental feature of spacetime we don’t understand? Why can’t gravity be reconciled with the better-understood quantum forces? All these questions remain unanswered. Many scientists think gravity must be generated by a massless particle, and have even dubbed it the graviton. But experiments to detect this entity (using a super-collider, for example) can’t be performed with current technology.