Mass and Energy are the same thing, Anon.
If Energy moves, it usually travels in a straight line, but Mass is when these spiraling bits of energy, begin to rotate around each other, rather than in a line.
They are still moving, but just around themselves, in an infinite loop.
The energy that makes up these loops still moves at exactly the same velocity, that never alters, but as these globs of mass accumulate, the rate of spiral decreases (conserves energy), as the distance it has to move increases.
The thing about spirals, is they can occupy the same volume, but if they are out of phase, they will never hit each other, and this forms a multitude of various shapes, but they are very stable.
If these looping spirals of energy, do collide, that is radiation, but it could take an enormous length of time for this to occur, or "half life".