[AnonNo15]

1) you start from Big Bang and go forward.

2) you can at any state check the difference between the simulated state and historical record

3) if your simulation converges, that is the closer to present day, the less the uncertainty, then you are on a right track.

4) if you simulation diverges you go to step 1 and tweak the initial conditions

5) you don't run a single simulation - you run many in parallel and select the best match at each step.