All the evidence that astrophysicists and cosmologists have to date tells us that the age of the universe is probably about 13.7 billion years, give or take. Now, this estimate must be taken with a grain of salt, because by universe, of course we mean the observable universe. That is an important qualification.
The general agreement as to the full dimension of the universe is an apparent contradiction of this estimate of its age, because it has been established that the distance in any direction from the Earth to its farthest observable boundary is about 46 billion light years. Of course, nothing can travel faster than light, so the explanation of how this can be should be considered in light of Hubble's law, and the factors concerning that time when the light that we are receiving from that boundary was first emitted, when the volume within that boundary was 1292 times smaller than it is now. You'll remember that Edwin Hubble discovered that the universe is expanding.
There is an awful lot of money invested in those highly sophisticated scientific methods that have enabled us to discover the information that we now have on these matters - billions of dollars, actually. It wouldn't do for one to be skeptical just because one doesn't understand how or why we know those things. We should be content to take what we do have in the way of this knowledge on faith.
The expanding universe suggests that there is a beginning. Because time, space, and matter are all factored into the equation of a dimensional (relative to before) point of origin that is to say, the beginning - we can assume that nothing existed before the event that spawned the creation process occurred, and because nothing is not something that we can walk around in, or wave our hand through, or poke at with a stick, the only conclusion we can draw is that it happened. Or, it happens, and that is when time and everything begins. I am of the considered opinion that this process of a perpetual beginning continues to happen at the periphery of the universe, which lies alomost inconceivably far beyond our limit of observation.
So we are left with the conclusion of sorts that some like to call the Big Bang Theory. It is certainly not wrong, given the evidence, so to suggest that the Big Bang Theory is a cosmic blunder is superfluous. But all that this means is that we can say with some certainty that a beginning occurred which was a moment in the evolution of this cosmos when things were quite different than they are now.
Now, the challenge posed by the title for the subject of this discussion is somewhat ambiguous. Does the title mean to suggest that the Big Bang itself is a blunder, or that the Big Bang Theory is a blunder? I will go with the latter and conclude that the title is not well thought out, and furthermore, encourage anyone who thinks that creation is a blunder to leave this universe immediately and/or forever hold their peace.