For those who may not realize, or missed it, what SelfAdjoint and Masudr are trying to convey is that there are MORE consideration to be taken into account than just "energy goes to matter". One needs to consider a number of conservation laws for something to occur. You just cannot take energy and BAM, convert it into matter. Pair-production, for example, where high energy photons converts into a matter-antimatter pair can only occur in the vacinity of "massive" objects due to conservation of momentum issues.
There are specific rules and laws for things to occur in physics in which there are not only qualitative, but also quantative restrictions. Knowing just ONE of the rules does not tell you if something is possible or not.
Zz.