Indra is invoked over 50 times in the 191 verses of the 1st Book of Rig Veda making Him the most powerful God of that time. Vishnu is only invoked 3 times.
Following the traditional belief that the Aryans established the Indus Valley Civilization, and pushed the original inhabitants – the Dravidians - farther South; trying to place Rig Veda in the historic context of the Indus Valley Civilization (excavated finds) raises the question whether Rig Veda and Vedic Gods were brought to India by the Aryans, and if so what was the religion of the original Indians in 5000 BC? Are the excavated seals depicting Gods, Aryan, and if so, are the Vedas, thus Hinduism, not indigenously Indian?
Irrespective of its origins Rig Veda is a masterful composition created by immense intellect with an amazing command of language and metre, a timelessly poignant Ode to God by men not content with fate, who called their mighty God, Indra.