Here's how I understand it (but I may be totally wrong so please correct me if this is wrong). Common knowledge is that there was an initial event called the B

Here's how I understand it (but I may be totally wrong so please correct me if this is wrong). Common knowledge is that there was an initial event called the Big Bang but there's no observational or theoretic evidence that there was such an initial event or explosion. Observations are limited by cosmological horizon, and our theories are unable to describe the state of the universe in its "early on" stage. What we do know however is that the universe is expanding and getting less dense and colder as a result. By going backward, we interpolate the state of the universe far in the past up to a stage where it was very dense and very hot "soon after" an hypothetical instant zero that we don't know happened. Also at that stage, the universe may have been infinite already (provided it's infinite now). Maybe it's better to think of the Big Bang has a past state of the universe rather than an "explosion" that happened at given time and place.