Where does the extra matter come from to a relativistic particle?

Question

Mass by definition is the amount of matter contained in a body. But for a relativistic object its relativistic mass is greater than the actual rest mass. If accelerating a body increases its mass then by definition it is increasing the amount of matter in a body.

So my question is where does this extra matter come from?

Answer 1

'Perceived mass' is just another name for energy, and the contribution to that from kinetic energy specifically depends on relative motion of body and observer.