When Einstein published his general theory of relativity in 1915, he didn’t have any evidence his idea was true. The math worked out, yes, and he had a few examples he could point to that seemed to agree with his theory, like the hard-to-explain orbit of Mercury, for example.
But it wasn’t until the total solar eclipse of May 29, 1919 that Sir Arthur Eddington and his two teams of scientists were able to confirm that, during the eclipse, a few stars seemed off-kilter, displaced from their usual spot in the sky. It was the first evidence that the mass of the sun had caused a bulge in spacetime, sending the starlight on a very slight detour. Einstein was on the front page of newspapers worldwide the next morning.