Gravitational lensing was how general relativity was "proven" back in 1919 [1] (4 years after publication). Einstein had predicted this weird effect but it was impossible to see in action because the only observable object massive enough to cause lensing was the sun and you cannot take a picture of it. In 1919 they managed to take a picture of the sun during a total sun eclipse which showed the effect clearly.

[1] https://www.discovermagazine.com/the-sciences/how-the-1919-s...