Does Big Bang Theory imply the creation of matter before that of radiation? Thus an original mass of a giant atom decays to produce radiation leading to the creation of matter in an expanding universe rather than its origin being a primary burst of radiation.
The Big Bang theory does not in itself need to focus on the distinction between matter and radiation. And the Big Bang theory is not a statement about origins in the philosophical sense. Rather it is a statement about the nature of the evolution of the universe from very early times. It holds that that evolution is one in which an initially hot dense state was so configured as to give rise to a universal expansion as everything moves further apart from everything else. Consequently the average distribution becomes gradually less hot and less dense as spacetime expands. In the early times the temperature is so high that matter in the forms we see around us now was not in existence, and this early state is more akin to radiation than it is to matter. But the Big Bang theory does not describe creation; rather it describes the evolution of a group of quantum fields, or, perhaps, a single multi-dimensional field, which already existed and had many specific properties, highly tuned and in a very special configuration.
What modern physics has to say about the origins of the universe is that, at the level of its basic constituents, the universe is now what it always has been, namely a specific highly precise set of fields, but the configuration or state of those fields has developed over time. You can call those fields either matter or radiation, but perhaps the best way to describe the situation is to call them both.
The Big Bang started from a singularity -- which is to say not a physical singularity, which would be an oxymoron. A singularity means a point at which we have no mathematically valid description of physics. General relativity implies nothing at a singularity, except that we need a more comprehensive theory to say what happened.
Going back as far as we can, the initial state was not a "giant atom" (Edouard may be right in suggesting you get this idea from Le Maitre). The earliest state we can describe consisted of large amounts of matter and antimatter together with radiation.