How do i slow down physics? I want to know how to slow down physics simulations or at least the explosion modifier.

I want to know this so i can render out the explosion with a high fps but over a longer period of time so i can slow it down and still keep the animation smooth.
If you are using Rigid Bodies, you can just decrease Speed in Rigid Body World settings under Scene tab:



You can also increase Step Per Second and Solver Itarations to get smoother and more detailed simulations.