It's not quite accurate to say there was a single year when the theory of relativity was "discovered." It's more accurate to say that Albert Einstein developed the theory of relativity in a series of steps over several years:

* 1905: Einstein published his paper on Special Relativity, which dealt with the relationship between space and time.

* 1915: Einstein published his paper on General Relativity, which extended Special Relativity to include gravity.

So, while 1905 is often cited as the year the theory of relativity was "discovered," it's more accurate to say that it was a process that unfolded over several years, with 1905 and 1915 being key milestones.