> In fact, relativity was only recently fully backed up with experimental data.

Can you elaborate on the assertion you made here? In addition to the important points @elbasti made about tests performed approximately a century ago, what does it even mean for a scientific theory to be "fully backed up"? Such theories can be tested and the tests either passed or the theory disproven but it's not possible to _prove_ such a theory. And to some extent we already know that relativity cannot be the final answer because it doesn't mesh well with quantum mechanics (which has been experimentally tested substantially, arguably even more than relativity has).