The theory, not the series.
According to an Associated Press poll, some of America's top scientists are "depress[ed] and upset" over the fact that a significant portion of the population expresses some doubt over what scientists consider to be "scientific facts". When it comes to the big bang theory around 51% of the population have some doubt. I have some doubts of my own.
If we look at it in the simplest terms, scientists have determined that the universe is expanding and have concluded that the universe must have had a beginning and they can calculate when in the past this event occurred. So far so good. They go on to theorize that the entire mass of the universe started out infinitesmally small and began expanding faster than the speed of light. This is where my doubt kicks in. I doubt if there is any human on the planet that can actually understand what this means in scientific terms.
We have been taught to believe that it takes infinite energy to accelerate mass to the speed of light, and also that time slows down for anything moving and, presumably, time stops when the speed of light is reached. These two things would seem to make it impossible for anything to exceed the speed of light. Science has a partial answer to these problems: Mass can move at or above light speed as long as it does not "communicate" with any mass it is moving away from at these speeds. Presumably this takes care of the infinite energy problem, although what is meant by "communicate" is hard to comprehend.
Time is another problem. If we assume science is correct in that time slows down as the speed of light is approached, what happens when the speed of light is exceeded? Was the universe going backwards through time in the beginning? What would that even mean? I suppose we could dismiss the time problem as being more of a problem for time theory in general and not specific to the big bang theory. I could accept that.
I still see a problem with the physics involved. Scientific understanding of physics seems to depend on understanding the "laws" governing atoms and subatomic particles and the relationship of these things to energy. I have no problem believing science has set a good foundation for understanding nuclear physics. The problem is, if all the combined mass in the universe started out smaller than an atom then this mass would not have been made up of atoms or subatomic particles as we understand them. In other words, the nature of this "mass" is beyond all scientific comprehension.
My conclusion: Many of the details of the big bang theory are faith based beliefs rather than valid scientific theories.