Every time Einstein's theories have been tested, they have supported his work. He himself said "No-one can ever prove me right - but one experiment can prove me wrong." Today is not that day.
It's been found that at the sub-atomic level, 95% of the mass of an atom is not mass, but energy and movement of sub-atomic particles:
This Theory of Special Relativity (not to be confused with Einstein's Theory of General Relativity - yes, there are two theories of relativity!) states, essentially, that mass and energy are interchangeable. The energy equivalence in a given mass is desribed by the famous formula above: it equals the mass times the speed of light squared - which is quite a lot! If one gram of matter was spontaneously converted to energy in an uncontrolled fashion, it would realise 898,755,178,736,817 joules, which would be an explosion more than 16 times as powerful as the Hiroshima atomic blast. Be careful with that matchbox...
I'm so proud of the human race that we can find out things like this. It's just astonishing to think that, at a level we will never directly perceive, 95% of the mass of every atom is just... energy and movement. (It goes to show you that quite a lot can be done by frantic arm-waving...)
Einstein was a truly remarkable man. I must admit to some mental reservations, however. His theory does not explain why when, on Mondays when I am tired and slow, I don't weigh any less than usual. Pfft.