The idea of relativity was first proposed by Einstein ant the age of 26 to answer a question that had plagued physics until that time. Physicists until then believed that light was a mechanical wave, much like sound. In order for this to be true there must be something everywhere to vibrate, afterall, waves are nothing more than the vibration of particles to tranfer energy from a source. So, they created the substance ether, and it was believed to cover the entire Universe. They used this idea to come up with the theory that the laws of mechanicsa are valid only in frames of referense in uniform motion. Einstein simply said that ether didn't exist. They then asked Einstein, what does light move at speed C reletive to? He replied, to anyone who cares to observe it. His idea boils down to one statement, "The laws of Physics are the same in all inertial reference frames." His later General Theory of Relativity goes to explain that the laws of physics are the same in all reference frames, even those accelerating. This came from the discovery of curved spacetime (which was not proven for many years). Some of the things to come out of the theory of relativity are curved spavetime, the twin paradox, the Lorentz transformations, and time dilation. Some of these topics are discussed in my other cool stuff section. So basically, the laws of physics are true throughout the Universe as long as you define a reference point from which to define a quantity.