What it means is that most professionals do not believe that the universe started with the big bang. That is why they use phrases like “the universe as we know it today”. We know this because of the observed smoothness of temperatures and densities including the Cosmic Microwave Background that the big bang was preceded and set up by a previous phase of the universe. This previous phase is generally but not universally believed to be “cosmic inflation”. It had an unknown length, perhaps lasting billions or trillions of years. We do not know when or how the previous phase started. But we know that it did exist because we can extrapolate backwards to the final fraction of a second of that previous phase, which is now included in the timeline of the standard model.
What it means is that most professionals do not believe that the universe started with the big bang. That is why they use phrases like “the universe as we know it today”. We know this because of the observed smoothness of temperatures and densities including the Cosmic Microwave Background that the big bang was preceded and set up by a previous phase of the universe. This previous phase is generally but not universally believed to be “cosmic inflation”. It had an unknown length, perhaps lasting billions or trillions of years. We do not know when or how the previous phase started. But we know that it did exist because we can extrapolate backwards to the final fraction of a second of that previous phase, which is now included in the timeline of the standard model.