One of the main problems with the Big Bang is that the temperature of the universe is nearly uniform. If a “big bang” event had created the universe, then according to some explanations, there hasn’t been enough time between then and now for it to have reached that temperature equilibrium. Here’s Zeeya Merali writing for Nature News:

To most cosmologists, the most plausible explanation for that uniformity is that, soon after the beginning of time, some unknown form of energy made the young Universe inflate at a rate that was faster than the speed of light. That way, a small patch with roughly uniform temperature would have stretched into the vast cosmos we see today.