Example. I tell you that I arrived at the park by driving my combustion engine vehicle along a particular route that took a certain amount of time. You agree that this is consistent with science. The next day I tell you that I arrived at the park by instantly teleporting half way to my magic carpet, which I piloted the rest of the rest of the way to the park. You begin to have doubts about today's account being consistent with physics.
Once upon a time, there were actual problems with the Big Bang being consistent with physics. Then Stephen Hawking fundamentally altered physics to account for the expanding universe we observe. If you haven't read his thesis, I recommend it. The "Big Bang" as a name/label, kind of fell out of use in deference to Hawking's "singularity." The model came about by the observation that the (observable) universe is expanding. If you go backward in time, the universe is contracting. The singularity is simply the theoretical limit. If you don't have a problem with the concept of absolute zero, or a perfect black body, even though you know they don't exist in nature, then you shouldn't have a problem with the theoretical point/limit whereby the universe began its expansion by simply tracing the expansion backwards. It's just a model. Many people have argued their own personal speculations about what happened at the "singularity" or thereabouts, and it's all irrelevant because nothing changes the model itself.
