If, at the moment of the Big Bang, "stuff" had no mass, how did it gain mass?

At the moment of the Big Bang, this "stuff" (without mass) "expanded" faster than the speed of light and Einstein said something with mass cannot travel etc, etc, etc.

"They" really don't know and "they" keep on wriggling things to suit themselve when it suits them.

"They" say Einsteins theory doesn't hold up for the moment of the Big Bang sooooooo, why do they keep putting things out as a fact when they're really only a theory? BBL