E=mc2

[Tldrknhndsm]

In nature how and where is energy converted to matter, where do the necessary conditions exist, is the process well understood, pure theory, or not understood at all?

 

[PeroK]

The relativistic energy-momentum equations, of which $E= mc^2$ is a special case, form the basis of all high energy particle collisions and dynamics. This includes particle scattering, creation and decay.

Most of the particles in the standard model are short lived and quickly decay into smaller particles, with the reduction in rest mass being released as kinetic energy.