There is now a large body of evidence to support the Big Bang Theory for the origin of the universe, but the problem remains as to the origin of the material or energy which initialised it. As the UK’s Astronomer Royal Martin Rees puts it
“We don’t know what banged, or why.”
And although much (but not all) of the theory behind Big Bang concept is backed up by observations, there remains a central and fundamental conflict with the modern physics that is used to explain it. Thus either modern physics theory has errors or omissions - or Big Bang theory does.
“The Big Bang theory, built upon the equations of classical general relativity, indicates a singularity at the origin of cosmic time; this infinite energy density is regarded as impossible in physics.” source
“The basic idea of the big bang is that the universe has been slowly expanding and cooling ever since it began some 13.7 billion years ago. This process of expansion and cooling explains many of the detailed features of the universe seen today, but with a catch: the universe had to start off with certain properties. For instance, it had to be extremely uniform, with only extremely tiny variations in the distribution of matter and energy. Also, the universe had to be geometrically flat, meaning that curves and warps in the fabric of space did not bend the paths of light rays and moving objects.”
This problem was countered with the 'Inflation' theory developed in the early 1980s - however :
“Highly improbable conditions are required to start inflation. Worse, inflation goes on eternally, producing infinitely many outcomes, so the theory makes no firm observational predictions.”
Source : Professor Paul J. Steinhardt, director of the Princeton Center for Theoretical Science at Princeton University, writing in Scientific American, April 2011