Question

Why did one-half of the Moon's surface remain

hidden from human sight until the era of space
probes, which started in 1959?

Answer 1

The moon's revolution around its axis and the moon's revolution around the Earth take the same time, approximately 27 days. That causes one side of the Moon to always face the Earth while the other side always faces away from the Earth. That is why it took probes to go into space to photograph the side that is never visible from Earth for people to be able to see it.