I shall bring one more desillusion, things are not like people usualy believe, there are some concepts that almost never are clearly explained and so people understand in a wrong way what it really are.
Mass is not matter, mass is a property of matter that mesures the amount of inertia that a matter has.
What is transformed into energy is not matter, what is trasformed into energy is the amount of inertia that a matter has, also energy can be transformed back into inertia.
When an atomic bomb explodes neither a single electron vanishes transformed into energy
.When the nuclear fission happens the uranium nucleous is divided in two giving two other elements. If we add the number of protons, neutons and electrons that the uranium atom has before the fision and we compare to the sum of protonts, neutrons and electrons of both elements produced by the fision and add the two liberated neutrons we shall find that are the same, no proton, no neutron, no electron has dissapeared, matter was preserved after the explosion.
So if no matter has vanished from where do come the energy liberated by the explosion?
The answer is that the energy stored in the uranium nucleous is bigger than the sum of energies stored in the nucleum of the produced elements.
 The energy stored in the nucleums we measure as mass and this is what is converted or liberated as energy, only a question of units.
Matter (protons, neutrons) is preserved by the explosion, a fraction of the internal energy (mass) is converted or liberated as energy.
Relativity tell us that mass increase with speed, but it doesn't mean that a plane flying becomes bigger than is on ground, it only means that a plane that is flying has more inertia than when is on ground.

Stupidity also evolves!