Einstein's Theory of Relativity.
In 1905, Albert Einstein determined that the laws of physics are the same for all non-accelerating observers, and that the speed of light in a vacuum was independent of the motion of all observers. This was the theory of Special relativity.
And what is General relativity? Essentially, it's a theory of gravity. The basic idea is that instead of being an invisible force that attracts objects to one another, gravity is a curving or warping of space. The more massive an object, the more it warps the space around it, thus attracting surrounding objects like planets and other astronomical objects in space. This theory also applies to everyday objects here on Earth.
So, Special relativity says that motion is always relative and that space and time exist in relation to each other. General relativity states that planets bend space and time the way a bowling ball bends a trampoline.
Embedded in his theory of relativity, is the famous equation, E = mc2
Einstein's greatest equation, E = mc2, is a triumph of the power and simplicity of fundamental physics. Matter has an inherent amount of energy to it, mass can be converted (under the right conditions) to pure energy, and energy can be used to create massive objects that did not exist previously.
So, in summary, the theory of Relativity has become the catalyst to our understanding of the properties and dynamics of our universe.