Minor nitpick: We definitely do not know whether everything is discrete. There are plenty of quantum mechanical phenomena that do not have discrete spectra (you can have light of any wavelength for instance (with some caveats at the extremes of the energy scales)). We also do not have theoretical or experimental proof that space-time is discrete at the Plank length-scale - all we know is that our current theories break at that scale.

See https://physics.stackexchange.com/questions/9720/does-the-pl... (the given answer is pretty great, but beware, the author is known for being a bit hostile in his non-physics opinions)