It's difficult to pick just one "most popular" discovery of Einstein's, as he made several groundbreaking contributions to science. However, the one that is arguably the most widely recognized and has had the most profound impact on our understanding of the universe is the theory of relativity.

Here's why:

* It changed our understanding of gravity: Einstein's theory of general relativity replaced Newton's theory of gravity, showing that gravity is not a force but a curvature of spacetime caused by mass and energy.

* It led to amazing technological advancements: GPS systems, which are used worldwide, rely on the principles of general relativity to account for the time dilation caused by Earth's gravitational field.

* It revolutionized our view of the universe: Einstein's theories explained phenomena like the bending of light around massive objects and the expansion of the universe, opening up new avenues of research in cosmology.

* It's a fascinating concept: The idea that time and space are not absolute but relative to the observer's motion has captivated the public imagination and inspired countless works of fiction.

While Einstein made other important discoveries, such as the photoelectric effect which won him the Nobel Prize, the theory of relativity remains his most enduring and influential contribution to science.