There is some evidence to support the domino theory. For example, after the Soviet Union annexed Estonia, Latvia, and Lithuania in 1940, many people feared that the Soviet Union would also annex Finland and Sweden. Similarly, after the communist victory in China in 1949, many people feared that communism would also spread to other countries in Asia, such as Japan and South Korea.
However, there are also examples of countries that did not fall to communism even after neighboring countries had fallen. For example, despite the fact that the Soviet Union annexed Poland and Czechoslovakia in 1948, Yugoslavia and Albania remained independent. Similarly, despite the fact that China became a communist country in 1949, Taiwan and South Korea remained non-communist.
Ultimately, the domino theory was not a perfect predictor of the spread of communism. However, it did capture the fear that many people had about the potential spread of communism during the Cold War.