14. The physics of dark matter is simple. We don't know exactly what the d.m. is, but it mustn't interact with matter (except through gravity) or we would see it. Hence the evolution of d.m. involves only gravitational force calculations. Baryonic matter can also cool, heat, shock, interact with radiation and magnetic fields etc. - it is much more complex to handle. In addition, due to cooling, it can form very dense clumps (e.g. stars). It is impossible to range from large scale structure down to such small objects in a single simulation.