There are a few areas where the Big Bang Theory has a bit of trouble. Mainly these problems have to do with the appearance of the Universe itself. The apparent "flatness" of the Universe is a anomaly that the big bang theory does not address by itself . Also there is the size of the Universe and distances between it's farthest points. The BBT does not adequately explain how these situations could arise.
To "answer" these , and other, questions a new theory was developed in the 1980's. It came to be known as The Inflation Theory. This new idea, basically, states that an extreme rapid expansion took place just before the "Big Bang". Although it oversimplifies the theory a bit....think of this rapid expansion as a growing bubble of matter and energy. Just before the "explosion" this bubble of matter and energy grew very large in relation to where it started. This is essence of the Inflation theory and when it is applied to the Big Bang theory it answers many of the questions that arose in the early days of the Big Bang.
The fact that the Inflation Theory has problems of its' own such as....What fueled the extreme rapid expansion of the bubble?... I will leave for another time.