The 20th century was full of shocks in terms of our knowledge of the universe. A little more than a century ago, we believed that everything we could see in the sky was contained within the Milky Way galaxy.
We used to believe that Newton's law of universal gravitation, which governs the universe, was static, unchanging, and possibly everlasting.
In just a few short years, all of that drastically changed. Newton's gravitation was surpassed by Einstein's General Relativity, which revealed the connection between matter and energy and the structure of spacetime.
His equations implied that the Universe couldn't be static and had to be evolving throughout time, which the discovery of the expanding Universe proved to be the case.
Additionally, his hypothesis foresaw the existence of black holes, which were eventually identified, detected, and even directly imaged.