U.S. physicists say they’ve detected gravitational waves for the very first time, marking a discovery that proves one of Albert Einstein’s last unverified theories about the universe.

Einstein theorized that gravitational waves are tiny ripples in the fabric of space-time created by all objects moving through time and space. Researchers from the Laser Interferometer Gravitational-Wave Observatory revealed Thursday that they’ve detected some of these ripples, created by the collision of two gigantic black holes.

“We did it,” LIGO laboratory executive director David Reitze said at an announcement at the U.S. National Science Foundation.

The discovery is expected to open up a whole new avenue for physicists to examine the nature and history of the universe, by allowing them to observe the gravitational properties of massive bodies in space.

Reitze compared the impact of the discovery to Galileo’s invention of the telescope.

“We’re opening a window on the universe,” he said.

Reitze said the gravitational wave was detected at the LIGO facility in Louisiana, on Sept. 14, 2015.

The discovery comes 100 years after Einstein first predicted the existence of gravitational waves, as part of his theory of relativity.

What is a gravitational wave?

The simplest way to grasp the idea of gravitational waves is to picture all of space-time as a big, stretchy trampoline. (Space-time, by the way, is our three-dimensional existence, plus time. Just as something can be located at an X, Y and Z coordinate in the three-dimensional world, it also has a time coordinate in space-time).

If all of space-time is like a trampoline, then a large object, such as the sun, is like a bowling ball weighing down a spot on the trampoline. For comparison’s sake, the Earth would be like a marble, spiraling in circles around the large depression made by the bowling ball (i.e. the sun). These objects send out gravitational waves as they move across space-time, like ripples moving through the fabric of a trampoline. They’re very, very minute, but they’re there, and scientists believe they’ve learned how to detect them.

But because the waves are so hard to detect, the LIGO researchers had to look for something that would make a massive wave, such as the collision of two black holes. It would be like putting two elephants on the trampoline at the same time.

How did they detect the waves?

According to Einstein’s theory, gravitational waves stretch and squeeze reality ever so slightly, so that we can’t even see it happening. However, light doesn’t play by the same rules, so it actually appears to warp as it travels past large objects – although it’s actually reality changing, not the light. Astronomer Arthur Eddington confirmed that element of Einstein’s theory in 1919, when he observed light “bending” as it travelled past the sun.

The LIGO project is based on this theory that a gravitational wave will bend all of reality, except light. The LIGO set up twin detectors in Livingston, La., and Hanford, Wash., with lasers shining down four-kilometre tunnels. Researchers then waited for a massive gravitational wave to pass through Earth, warping space-time ever so slightly, and thereby changing the relative distance traveled by the lasers. According to Einstein’s theory, the change would be incredibly minute, so the instruments had to be very precise in order to detect and measure it.

Is it for real?

The scientific community has already had a false positive when it comes to detecting gravitational waves, so scrutiny of this new discovery will be intense.

In 2014, a team of Harvard researchers claimed to have detected gravitational waves triggered by the so-called “Big Bang” that theoretically started the universe. However, that discovery was debunked early last year, when closer analysis revealed that cosmic dust was responsible for the phenomenon.

On Thursday, Reitze said his team is convinced of the accuracy of the evidence they’ve collected.

“It took us months of careful checking, rechecking, analysis, (and) looking at every piece of data,” he said. “We’ve convinced ourselves.”