Scientists still talk about gravity as a theory because they can’t explain exactly how it works. Albert Einstein hypothesized gravity waves, but there was no way to confirm they existed… until recently.
Granted, the science is a bit hard to comprehend because all the numbers are extreme one way or another, but basically, over a billion years ago and about a billion light years away, two massive black holes collided. Recently two incredibly precise laser detectors in Washington and Louisiana detected the resulting disturbance, and after much checking Einstein’s theory appears validated.
Of course, that’s the simplified explanation. The detectors have to be extremely sensitive because by the time that wave has traveled a billion light years to reach us, it’s barely detectible. The resulting “stretching and squeezing” of space would be less than an atomic diameter. That’s why two separate detectors are needed. Any vibration, a passing truck or even a gust of wind, might trigger a false signal.
The detectors are a story in themselves. The system is called LIGO for Laser Interferometer Gravitational-Wave Observatory. They consist of four-kilometer-long tunnels in the shape of an L that contain laser beams about 50,000 times more potent than a standard laser pointer. The light bounces back and forth between very smooth mirrors, while any difference in the time taken to make the trip is measured. The detectors were first put on-line in 2002, taken off-line in 2010 for upgrades, and the newer, more sensitive LIGO went back into operation in the fall of 2015. Two days later (!), both detected a disturbance within seven milliseconds of each other. It’s a seminal moment in the history of physics.
There’s an excellent article about the success in the April 2016 issue of Smithsonian magazine (“Catching a Wave”, page 25). And by the way, the cost of all this has been over a billion dollars.