What are the simplest things we know about the Big Bang?

We know that the galaxies out to the most distant ones we can discern, are moving away from us so that the universe is 'expanding'.

We know that the universe is bathed in the microwave glow of the fireball radiation that accompanied the Big Bang.

We know that there is a 'universal' abundance of hydrogen, helium, deuterium and lithium which doesn't vary much in no matter which old objects we look at.

We know that there are no stars older than 20 billion years even though these kinds of stars should be easily recognizable if the universe were truly older than 20 billion years.

These are the basic facts we get to work with. There are also a number of other observational facts that are also important, but not as irrefutable as the ones above for the idea that the universe had a birth a finite number of years ago, in a very hot phase, and is now expanding.