Celebrated physicist Stephen Hawking knows more about the universe than almost any other person ever to walk the planet, but some answers still escape even him.
When asked by ABC News' Diane Sawyer about the biggest mystery he'd like solved, he said, "I want to know why the universe exists, why there is something greater than nothing."
Until he stepped down last fall, he held the post of Lucasian professor of mathematics at Cambridge University, a position once held by Sir Isaac Newton, the "father of physics" himself.
During Hawking's 30 years in the post, he gave his colleagues new ways to look at the universe. But he also gave the public, through his many books and occasional appearances, a way to connect with the often obscure world of theoretical physics.
"Stephen has had a tremendous impact on our understanding of the universe, particularly of warp space and warp time in the universe, black holes, the origin of the universe," said physicist Kip Thorn at last week's World Science Festival event honoring Hawking.
Hawking's most well-known book, "A Brief History of Time," sold more than 9 million copies and is an international best-seller. He's even made brief cameos in pop culture staples like "The Simpsons" and "Star Trek."
But exploring the origins of time inevitably leads to questions about the ultimate origins of everything and what, if anything, is behind it all.
"What could define God [is thinking of God] as the embodiment of the laws of nature. However, this is not what most people would think of that God," Hawking told Sawyer. "They made a human-like being with whom one can have a personal relationship. When you look at the vast size of the universe and how insignificant an accidental human life is in it, that seems most impossible."
When Sawyer asked if there was a way to reconcile religion and science, Hawking said, "There is a fundamental difference between religion, which is based on authority, [and] science, which is based on observation and reason. Science will win because it works."
Hawking, 68, was diagnosed with the motor neuron disorder ALS, or Lou Gehrig's disease, when he was 21. He was told that he likely would not live more than a few years.
But on his personal website he said that with "a cloud hanging over my future, I found, to my surprise, that I was enjoying life in the present more than before."
"I suddenly realized that there were a lot of worthwhile things I could do if I were reprieved," he said.
He went on to have a family (he has three children and one grandchild) and, though his condition progressed, he continued his research unabated with the help of a wheelchair and, later, an electronic speech synthesizer.
Though Hawking no longer holds his position with Cambridge University, he continues to hold the public's attention.
In April, he grabbed headlines after the premiere of the Discovery Channel series, "Into the Universe With Stephen Hawking."