For one reason or another (for instance, a heart attack), a person's vital functions cease. They lose consciousness and their heart stops beating - they are clinically dead. During this time. they experience the sensation of coming out of their body, and although their body is unconscious, they are aware of people around them trying to resuscitate them. Then they begin to travel down a dark passage towards a brillant light. However, at some point they encounter a barrier, and they can go no further down the passage. They find themselves rushing back to their body, just as they are being "brought back to life".Uusally the experience is very real to the person, and it profoundly impacts their understanding of death.
I have heard this pattern repeated many times, by people from many different cultures. Other details often vary, and certainly different researchers come to different conclusions. But the core details are so consistent, it seems to me hard to deny that there is some sort of existence after death.
Although not a direct proof of the existence of God, this would certainly indicate that there is a spiritual realm - that we are more than just animated bodies.