On May 29, 1919, a solar eclipse forever altered our conception of gravity, rewrote the laws of physics and turned a 40-year-old, wild-haired scientist into a global celebrity - the very personification of scientific genius.
It was a very good day for Albert Einstein.
The 1919 eclipse across South America and Africa provided direct evidence for Einstein's mind-bending theory of gravity. He proposed in 1915 that gravity isn't a spooky force acting across space but rather is a feature of the essence of space and time. Gravity is the warping and curving of the fabric of the universe.
Einstein's theory - the general theory of relativity - was hailed by the physicist J.J. Thomson as "one of the greatest achievements of human thought." It has been confirmed by many more observations over the century, including the detection of gravitational waves and the first picture of a black hole just this year. He cracked a fundamental code of the universe.
And yet: Something's amiss.
Although Einstein seemed to have the final word on how the universe is put together, more recent probing of deep space as well as the inner workings of atoms have found places where the theory breaks down.
For example, inside a black hole, Einstein's equations suggest that matter and energy become so compressed they reach infinite density. But what does that mean? The theorists suspect it means they need a better theory.
"You can't calculate anything beyond that point, once the numbers become infinite. You've lost all control," says Emil Mottola, a theoretical physicist at Los Alamos National Laboratory. "That doesn't tell you that nature can't do that, but it's very suspicious."
The same problem applies when cosmologists rewind the film reel of the universe's expansion for the past 13 billion years and reach the very beginning of time and space, the so-called big bang. Einstein's theory doesn't quite work at the creation.
Notoriously - at least among theoretical physicists - general relativity doesn't explain how gravity works at the tiniest of scales, the realm of subatomic particles.
Dark matter has never been directlyobserved, but its existence is inferred through its gravitational effects, such as on the motion of stars in galaxies. It could be some kind of modification of the gravitational force that wasn't predicted by Einstein, said Lee Smolin, a theorist at the Perimeter Institute.
Dark energy, another cosmic mystery, is whatever is driving the acceleration of the expansion of the universe. This seeming anti-gravity acceleration was detected only in the late 1990s and strongly suggests that the universe will expand forever. So why is this happening?
"We don't know. That's why we call it 'dark energy,' " says Gabriela Gonzalez, a professor of physics and astronomy at Louisiana State University.
"I think there are plenty of mysteries that I hope to see the solution to in my lifetime," she said. "All these things need theories that can be confirmed by experiments. They need theories that have predictions."
Which brings us back to Einstein, and the eclipse.
Einstein had emerged from obscurity in 1905 with a series of astonishing papers that obliterated classical notions about time and space. But his greatest achievement came a decade later, in 1915, when he described the equations governing gravity. He'd figured out a fundamental feature of the universe, using merely the power of his brain. But was it true? What if his equations were just a mathematical fancy, something that looked nifty on paper but did not correspond to physical reality?
Einstein proposed an experimental test. A solar eclipse would block the sun's light and allow scientists to study starlight passing close to the sun. His theory predicted that the sun's gravitational field would displace the starlight by a certain amount compared with where it would be under classical theories of gravity.
British astronomer Arthur Eddington led an expedition to observe the eclipse from two locations, one in Brazil and one on the island of Principe near the Equatorial Guinean coast.
Einstein's theory was supported.
When the astronomer royal, Sir Frank Dyson, announced the results in November of that year, newspapers ran front-page stories and Einstein became famous all over the planet.
In his biography of Einstein, author Walter Isaacson recounts an exchange between Einstein and a graduate student, Ilse Schneider, when news came that the theory had been upheld. She asked him, she later recalled, what he would have thought if the eclipse observations had contradicted his theory.
"Then I would have been sorry for the dear Lord; the theory is correct," Einstein said.
Mottola notes that, since the days of Euclid and Aristotle, space and time had been seen as a passive stage for the events of the universe, unaffected by the comings and goings of planets and stars. But Einstein said that wasn't so: Space and time were affected by matter, and even light has to obey the geometry of curved space.
The modern world depends on accepting this cosmic truth. Spacecraft trajectories have to take general relativity into account. So does GPS. So does military targeting.
Einstein's theory carried astonishing implications for exotic things in the universe, not least of which are black holes. Perhaps the most thunderous modern confirmation of Einstein came with the detection of gravitational waves from colliding black holes. Einstein had predicted the existence of gravitational waves; a century later, scientists found them.
The universe has not run out of surprises, and theoretical physicists remain in business. The questions don't tend to get easier to answer over time. When Mottola is asked about what happened, exactly, at the beginning of the universe, he says, "Sometimes you have to say you don't know."