The roots of Hinduism have been traced back to the Indus Valley Civilisation and the Indo-Aryan culture.
Indo-Europeans extended the name of the province of Sindh to the whole country lying across the Indus river. The inhabitants were simply called Hindus.
Hinduism was not founded at one moment. It is a complex religion that has evolved over many millenniums, but has been dated to about 2500BC.
It has been estimated that there is between 800 million and 1 billion Hindus living in the world today. About 79% (750 million) of India's population is Hindu.