But despite the triumph of Einstein’s theory — general relativity — physicists still wonder whether it will someday face the same fate as Newton’s law. While Einstein’s gravity has passed every test so far, nobody knows for sure that it applies everywhere, under all conditions. In particular, there is no guarantee that general relativity reigns over the entire expanse of the cosmos. And several rival theories have been proposed over the years just in case it doesn’t.

After Einstein proposed his new theory, it was mostly ignored for a few decades. But in the last half of the 20th century, general relativity became the theory of the universe. Its equations describe the expansion of the cosmos from its initial high-density, hot big-bang beginning to its current rapidly accelerating expansion. And today general relativity has earned increasing popular notoriety as scientists have verified its more exotic predictions, including black holes and the vibrations in space known as gravitational waves.

But general relativity’s string of successes may not be endless. It’s true that the theory (along with the theory for nature’s three other fundamental forces) describes the observable universe quite well. That description includes massive amounts of invisible mass, known as dark matter, along with a peculiar repulsive force, called dark energy, perfusing all of space. But the dark stuff’s existence is deduced from the assumption that general relativity is correct.

“Given that there is no other (nongravitational) evidence for the dark sector, it is a matter of common sense to question some of the fundamental assumptions that go into the evidence. And the main assumption is that general relativity is the underlying theory of gravity,” astrophysicist Pedro Ferreira of the University of Oxford in England writes in the current Annual Review of Astronomy and Astrophysics. If you don’t assume general relativity is in fact correct, then “evidence for the dark sector may signal a breakdown of general relativity on cosmological scales,” Ferreira points out.

In other words, it’s conceivable that there is no dark stuff. If that’s the case, apparent evidence for its existence might actually be a sign that the true cosmic theory of gravity differs from Einstein’s. If so, the current picture of the cosmos would have to be drastically redrawn.

Still, physicists have plenty of reason for confidence in general relativity’s reliability. For one thing, it solved a knotty problem that had perplexed astronomers about the planet Mercury: a discrepancy in its orbit from that forecast by Newtonian gravity. Einstein announced his theory in 1915 as soon as he was able to show that it correctly predicted Mercury’s actual orbit.

Einstein’s key to solving the Mercury mystery was conceiving gravity as an effect of the geometry of space (or technically, space-time, since his earlier work had shown space and time to be inseparable). Gravity is not a mutual tug of massive objects, Einstein said, but rather the result of a mass’s distortion of the space-time surrounding it. Objects orbit or fall into a massive body depending on how strongly the space-time around it is curved. Rather than responding to some attractive force, masses just follow the contours of space-time’s geometry.

Gravity as geometry led to the famous prediction verified in the 1919 eclipse. Einstein pointed out that the curvature of space-time near the Sun would cause light from distant stars to bend when passing nearby, changing the stars’ apparent positions as seen from Earth. That prediction inspired an eclipse expedition to the West African island of Principe in May 1919, led by British astrophysicist Arthur Eddington. Eddington’s team found that the positions of several stars were shifted by just the amount that Einstein’s math indicated they should be, and twice as much as Newton’s law predicted. When the eclipse team announced the results in November 1919, one news account heralded them as signaling the need for “a new philosophy of the universe.”