Moses did not tell us what God's back looked like. But we do know that an amazing thing happened to Moses after he saw it. Moses' face began to glow brightly. It was radiant.
When Moses came down from Mount Sinai, and the people saw that the skin of his face was shining like a bright light, they were afraid to come near him. Moses had to coax them to come hear his message from God.
When Moses finished speaking to them, he put a veil over his face. Then the people were not so afraid to have him walk among them. But whenever God came to speak to him, Moses took off the cloth covering his face. And when he came out and told the Israelites what he had been commanded, they saw that his face was radiant. Then Moses would put the veil back over his face.