Answer:
Many people own a copy of me. Without me the world would fall.
I am black and white. I have strings I have keys. I make sound without my lips. I make dough with no flour
Lovely and round I shine with pale light grown in the darkness a lady's delight.
I travel in a gaggle.
I run up and down the stairs without moving
It weighs next to nothing but no one can hold it for long. What is it?