Because I have read numerous books written by research cosmologists, and there are two sources in particular that I have relied on for this, and one is Leonard Susskind, who's a research scientist who covered this in one of his books on cosmology, and the other is from a Pew research poll of scientists taken a while back. "Infinity". With infinity, there simply is no "primary cause" since it basically states that there may have been always something, whether they be sub-atomic particles, energy waves, or whatever. And let me repeat, we know our universe works out mathematically, and we also know that the use of "infinity" in some equations does work out mathematically as well, therefore it has to be considered as a logical possibility or the equations simply would not work out. Finally, the concept of a "primary cause" is contrary to logic as a sole argument in that it has to beg the question as to what caused the "primary cause"? If one says that the "primary cause" was always there, then how is that any different than possibly accepting that "infinity" maybe was always there? Why is one supposedly logical but the other not? As for me, I just keep saying "I don't know".