The "objects" are made of "stuff" with mass, it must therefore have had mass "in the beginning".   I don't think anyone has "explained" yet how something without mass can, after travelling through "space" somehow gain mass?   If it IS possible for something without mass to gain mass, how come all the "stuff" without mass hasn't yet gained mass?   You see, when people can't explain something, they fall back on "it happened". Failing that, it's on with some other "invention" to explain away the fact that "they" don't know.

Where does it say something without mass gained mass?

I don't know much about this, but as far as I understand at the beginning there was infinite density. Density = Mass/volume  and since volume was zero a finite amount of mass over zero volume would produce this infinite density.  Then give it some volume through sudden expansion and you have mass (mass = volume x density).

??